RE: [AMRadio] Mike Cable Capacitance
I don't think I will spend much time trouble shooting, since I went to a whole new setup, I am now going balanced between the mike preamp (berringer) and the power amp (symetrix and soon a samson). The old nikko power amp had unbalanced inputs. I doubt it was rf getting into the audio chain, but its possible. I was quite sure things were much better after just changing the rg8m out, I had put phono ends on it, and had an adaptor (phono to mono 1/4 inch jack) into the berringer. That should have unbalanced the output of the berringer, which is supposed to compensate for the change in level output with some magic circuit. There were (are) no built in bits in the cables, I DID have an T pad (600 ohm I think) to attenuate one input on the nikko. The new amps have level controls for each channel, so I chucked that. I was also driving two inputs with one (berringer) output. I just thought, since BOTH nikko inputs were bypassed with .047uf caps, that is almost a .1!!! The wrong cable types, add on T pads, excessive bypassing, running two inputs off one output, lots of not so great things that can cause problems. Its good that I have cleaned up this mess. Another nice thing with the new amps is that they have headphone outputs, so I can hear the audio feeding the modulators without doing anything with the 8 ohm outputs. Brett N2DTS
Re: [AMRadio] Mike Cable Capacitance
Hmm, so the mike cable isn't the problem, it's the cable from the preamp to the modulator. I can't believe that any commercial audio gear would have such high output impedance that fifteen feet of cable capacitance would make much difference. And I don't think that the difference between fifteen feet of RF cable and fifteen feet of audio cable would make much difference at all. Now, musicians with high impedance guitar pickups can often tell the difference between guitar cables that are not much longer than 15 feet... and it does affect the highs. But a guitar pickup is a tricky, high impedance, highly inductive signal source. It seems more likely to me that maybe the preamp has a floating balanced output, and only one side is connected to the shielded line, and the shield is grounded. Output in that situation would be very quirky, and dependent on the load impedance and any bypass capacitance. If you had, say, 100pF to ground on either side at the preamp balanced output, and 100pF to ground at the modulator input (and I assume that the modulator input is single-ended), then it would act like a capacitive divider again, with 200pF effective capacitance to a low-z audio source. The resistive load would affect the low end, and varying cable capacitance would affect the level and the low frequency corner as well. However, if the bypass caps were 4700 pf or .01 uF, the effect of cable capacitance would be slight. For a few hundred pF of cable capacitance to affect the top end much, there would have to be only a very small amount of extra bypass capacitance. Maybe someone made a passive low-pass filter at the preamp output, and the extra cable capacitance is having a big effect on an unintended resonance. (Doubtful.) Or maybe there is some RF getting in that is getting rectified and causing an audio resonance when you transmit. Different cable, even a different cable path, can affect RF leakage. Do the RG cable and the Radio Shack cable have their own connectors, or did you replace the RG with the Radio Shack cable using the same connectors? And does the preamp use those 3-pin XLR connectors? That would suggest a probable balanced audio output. I really suspect something like a balanced output (and/or input) that is not connected right. To use a floating balanced output for a single ended system, just ground one side and take output from the other. Balanced windings are usually not center-tapped, but they might be set up for phantom power somehow... which is another possible source of odd effects if the winding is not connected right. The other likely possibility is RF rectification. I would run a variable oscillator through the preamp and measure response and output at various points with a VTVM or a modern digital meter, and see where the weird stuff is coming from. That's the best thing. Or you could feed some music source into the preamp, put the transmitter on a dummy load, and listen with a receiver while you change stuff. The differences you saw were large, and you should be able to track them down. Also, just try it as-is, but first with the transmitter on an antenna, and again on a dummy load, and see if that makes a difference. Could be RF rectification. Bacon, WA3WDR
Re: [AMRadio] Mike Cable Capacitance
A 15 foot cable with 30 pF per foot would present a 450 pF capacitance to ground, and this would work with the piezo microphone's effective output capacitance to act as a capacitive voltage divider, which would cut the mike level about in half (-6dB), or about 100% to about 50% modulation, which is about what was observed. The effect of this capacitive voltage division should be uniform across the audio frequency range, but the load resistance causes another effect at the low frequency end.. The total effective series capacitive reactance would be equal to the effective microphone output capacitance and the cable capacitance in parallel, or about 950 pF, so in this example the presence of the 15 foot cable would reduce the mike level by about 6 dB, and the relative low frequency response with any given load resistance would be extended down by about an octave. That can be a dramatic response difference if the low frequency cutoff is in the 300Hz range. I seem to recall reading, maybe something from Astatic, to the effect that a reasonable length of shielded cable will have little effect on the frequency response and output level of a crystal mike as opposed to other high impedance sources such as resistive networks and high impedance transformer secondaries. An interesting alternative with the D-104 is shown in the paperwork they used to include with the microphone, using a pushpull mic preamp. The cable is shown to be a shielded pair of wires. Each terminal of the mic element is connected to one of the wires, and at the amplifier end, each goes to one grid of a pushpull stage, each of which has its own grid leak resistor of 5 megohms or so to ground. The pair of grid resistors acts as a voltage divider with the common connection grounded. This puts 180 degree out-of-phase audio on each of the two grids. The pushpull preamp stage can be followed by a coupling transformer or R-C coupled to another pushpull stage. Astatic suggests a 100% pushpull amplifier, from mic preamp to final audio stage. This arrangement should be possible with any crystal mic element that has two ungrounded terminals like the D-104, as opposed to elements with one side grounded to a shielded metal enclosure. -k4kyv _ There are now three new levels of MSN Hotmail Extra Storage! Learn more. http://join.msn.com/?pgmarket=en-us&page=hotmail/es2&ST=1
Re: [AMRadio] Mike Cable Capacitance
A 15 foot cable with 30 pF per foot would present a 450 pF capacitance to ground, and this would work with the piezo microphone's effective output capacitance to act as a capacitive voltage divider, which would cut the mike level about in half (-6dB), or about 100% to about 50% modulation, which is about what was observed. The effect of this capacitive voltage division should be uniform across the audio frequency range, but the load resistance causes another effect at the low frequency end.. The total effective series capacitive reactance would be equal to the effective microphone output capacitance and the cable capacitance in parallel, or about 950 pF, so in this example the presence of the 15 foot cable would reduce the mike level by about 6 dB, and the relative low frequency response with any given load resistance would be extended down by about an octave. That can be a dramatic response difference if the low frequency cutoff is in the 300Hz range. I seem to recall reading, maybe something from Astatic, to the effect that a reasonable length of shielded cable will have little effect on the frequency response and output level of a crystal mike as opposed to other high impedance sources such as resistive networks and high impedance transformer secondaries. An interesting alternative with the D-104 is shown in the paperwork they used to include with the microphone, using a pushpull mic preamp. The cable is shown to be a shielded pair of wires. Each terminal of the mic element is connected to one of the wires, and at the amplifier end, each goes to one grid of a pushpull stage, each of which has its own grid leak resistor of 5 megohms or so to ground. The pair of grid resistors acts as a voltage divider with the common connection grounded. This puts 180 degree out-of-phase audio on each of the two grids. The pushpull preamp stage can be followed by a coupling transformer or R-C coupled to another pushpull stage. Astatic suggests a 100% pushpull amplifier, from mic preamp to final audio stage. This arrangement should be possible with any crystal mic element that has two ungrounded terminals like the D-104, as opposed to elements with one side grounded to a shielded metal enclosure. -k4kyv _ Get a FREE online virus check for your PC here, from McAfee. http://clinic.mcafee.com/clinic/ibuy/campaign.asp?cid=3963
RE: [AMRadio] Mike Cable Capacitance
I use an outboard audio amp for my 75A-4 receiver with a system of adaptors that requires no modification to the receiver itself. I pull audio directly from the grid pin of the 12AT7 1st audio stage (with the tube removed). This is a high impedance point, 470k if I recall correctly. To avoid high frequency loss in the 5 ft. or so of shielded cable between the tube socket and amplifier, I built up a cathode follower stage to replace the 12AT7. It is a plug-in module that plugs right into the 12AT7 socket, and gets its B+ from the screen grid pin of the 6AQ5, which was also removed to conserve drain on the power supply and reduce unneeded heat in the receiver. I can remove the plug-in units and replace them with the original tubes, and the receiver will be completely stock once again. -k4kyv _ Learn how to choose, serve, and enjoy wine at Wine @ MSN. http://wine.msn.com/
RE: [AMRadio] Mike Cable Capacitance
Pardon me guys I guess I getting old. Evidently I am no wording this properly. Bret said, that the cable in question was not the microphone cable but instead a cable that connects a preamp to a power amp and this should be much lower impendence in and out than a ceramic or crystal microphone circuit. In my last message I said or tried to say The microphone circuit is not in question. 73 John, WA%BXO -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Bob Bruhns Sent: Thursday, January 15, 2004 11:24 AM To: amradio@mailman.qth.net Subject: Re: [AMRadio] Mike Cable Capacitance Hi John, Crystal and ceramic mikes (piezoelectric mikes) generally act like a combination of a relatively low impedance source (I don't know exactly, a few thousand ohms or so), coming through a series capacitance of roughly 500pF. This tiny internal effective output coupling capacitance is the reason that we need a very high load resistance to maintain the low frequency response. A 15 foot cable with 30 pF per foot would present a 450 pF capacitance to ground, and this would work with the piezo microphone's effective output capacitance to act as a capacitive voltage divider, which would cut the mike level about in half (-6dB), or about 100% to about 50% modulation, which is about what was observed. The effect of this capacitive voltage division should be uniform across the audio frequency range, but the load resistance causes another effect at the low frequency end.. The total effective series capacitive reactance would be equal to the effective microphone output capacitance and the cable capacitance in parallel, or about 950 pF, so in this example the presence of the 15 foot cable would reduce the mike level by about 6 dB, and the relative low frequency response with any given load resistance would be extended down by about an octave. That can be a dramatic response difference if the low frequency cutoff is in the 300Hz range. Bacon, WA3WDR - Original Message - From: "John Coleman" <[EMAIL PROTECTED]> To: Sent: Thursday, January 15, 2004 11:08 AM Subject: RE: [AMRadio] Mike Cable Capacitance One of us, and it very well could have been me, must have missed something. I though Bret, N2DTS, said that the coupling problem (loss of audio level and high frequency response) was the cable from the preamp to a power amp using line level coupling. I was only mentioning that a high Z input, such as an old style XTAL mic input on the power amp would be a typical problem as you mentioned. But I don't think he is using that scenario. I think Bret said that he had a preamp for the mic and that it was the coupling cable from the output of the preamp to a power amp line input that was in question and that replacement with a very short cable increased the audio level and high frequency response. This doesn't seem very normal to me, that changing from a 15 ft to a 1 ft cable at normal line level medium impedances would make that much difference in frequency response and especially a noticeable gain difference form 20% modulation to 100% or more. A friend of mine is noted for placing resistors and capacitors inside a cable connector for the purpose of EQ or attenuation, rather than modifying the equipment. You have to be careful about using just any old cable in his place; it may have a 1 meg resistor in series with the cable built into the connector end. HIHI. John, WA5BXO -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Jim Bromley Sent: Thursday, January 15, 2004 4:06 AM To: AM Radio List Subject: [AMRadio] Mike Cable Capacitance John Coleman, WA5BXO, posted: (Regarding 50-ohm coax attenuating the high audio frequencies when used as a microphone cable). > I feel there must be some other thing that is overlooked > about the connector or cable. 15 ft of cable would not > normally cause any noticeable effect on audio unless the > terminating Z was greater that a few mega ohms as it would > be for some of the older equipment made for Xtal mike input > but I can't imagine a modern day line input being greater > than 100K ohms The impedance to be concerned with in this case is not the terminating impedance, but rather the source impedance of the microphone itself. In the case of the D-104, although the impedance of the microphone is specified at a nominal 10K-ohms, its actual source impedance is much higher. It is a well-known fact that the element must be terminated in a resistive impedance of at least 10-Megohms to obtain adequate low-frequency response - indicating a source impedance of at least that amount. Additionally, since there is no DC path through a crystal acoustic transducer, one would be suspicious that whatever the actual source impedance is, it would contain a capacitive reactance as a series component. The best solution to the problem i
RE: [AMRadio] Mike Cable Capacitance
I replaced a 10 or 15 foot piece of rg8m with a 20 foot radio shack regular audio cable. This went between the berringer on the desk, to the nikko power amp in the rack (now empty and out of the way). The rack will hold the homebrew pair of 4d32 rig in the future. The level was the same except for the extreme highs, which jumped from 20% modulation to close to 100% with the radio shack cable. This was from making loud s sounds into the mike. Doing more checking, I found I had put bypass caps in the audio power amp, both on the input, .047 uf, and the 8 ohm output .01 uf. These were likely to large, and along with the coax, reducing the highs. There was a large change between the coax and the regular audio cable when I tried that first, but its very likely it interacted with the bypass caps and pushed the roll off frequency down to 3000 to 4000 hz??? I changed the caps to .003 in that amp, and also bought 2 new amps, a one rack mount space 20 watt per channel symetrix amp, that seems to need no RF bypassing at all, and a samson 60 watt per channel amp, also one rack mount space (nice and small). It was mail order, and has not got here yet. The 20 watt symetrix (pawn shop special) works fine on the 811a mod deck, and the 4x150a mod deck (ab1), but does not have enough power to drive the push pull parallel 100th mod deck, it gets up to about 90% mod at the symetrix clipping (20 watts). I likely need to change taps on the driver trans to get more voltage, 4 100th tubes should take less than 20 watts drive... My ears are no good, but I think it sounds much better than it used to, much less high boost for the same sound (in my ears). I guess the 120 watt per channel nikko amp will be a ham fest special, its worked great for 20 years of abuse, has nice analog meters on it, but its quite large and heavy for the shack. I like having all the audio stuff close together on the desk, the berringer mike amp/compressor/eq/etc, the power amp, the tape deck, and the receiver audio amp (marantz). With a one rack space berringer, and a one rack space audio amp or two, you can drive a bunch of modulator grids in a very small space. High quality audio in about 4 inches of total rack space! I paid $90.00 for the symetrix, $180.00 for the (new) samson 60 watt per channel amp. And yes, I still have the good tube audio in the 32v3 transmitters, so I am not a total sell out. I hope to build up a nice tube mike and line preamp/mixer/eq/ compressor/power amp to drive the various homebrew rigs. A tube compressor that works well might be a bit tough though. I have bunch of nice balanced 600 ohm to grid hi fi type transformers that need to go in something. A pair of KT90 tubes will do 160 watts in AB1, very low distortion. Tone control circuits are in the back of the RCA receiving tube manual, along with the mike preamp and mixer circuits. That and the homebrew RF vfo/exciter/driver are all that is left to have a completely home brew station from mike jack to antenna. (can you home brew a mike?) I heard the first part of the article on the second homebrew receiver is in Electric Radio this month, but have not got my copy yet. I have no idea what pictures they used, I sent in a whole mess of photos of various things in the shack. Brett N2DTS
Re: [AMRadio] Mike Cable Capacitance
Hi John, Crystal and ceramic mikes (piezoelectric mikes) generally act like a combination of a relatively low impedance source (I don't know exactly, a few thousand ohms or so), coming through a series capacitance of roughly 500pF. This tiny internal effective output coupling capacitance is the reason that we need a very high load resistance to maintain the low frequency response. A 15 foot cable with 30 pF per foot would present a 450 pF capacitance to ground, and this would work with the piezo microphone's effective output capacitance to act as a capacitive voltage divider, which would cut the mike level about in half (-6dB), or about 100% to about 50% modulation, which is about what was observed. The effect of this capacitive voltage division should be uniform across the audio frequency range, but the load resistance causes another effect at the low frequency end.. The total effective series capacitive reactance would be equal to the effective microphone output capacitance and the cable capacitance in parallel, or about 950 pF, so in this example the presence of the 15 foot cable would reduce the mike level by about 6 dB, and the relative low frequency response with any given load resistance would be extended down by about an octave. That can be a dramatic response difference if the low frequency cutoff is in the 300Hz range. Bacon, WA3WDR - Original Message - From: "John Coleman" <[EMAIL PROTECTED]> To: Sent: Thursday, January 15, 2004 11:08 AM Subject: RE: [AMRadio] Mike Cable Capacitance One of us, and it very well could have been me, must have missed something. I though Bret, N2DTS, said that the coupling problem (loss of audio level and high frequency response) was the cable from the preamp to a power amp using line level coupling. I was only mentioning that a high Z input, such as an old style XTAL mic input on the power amp would be a typical problem as you mentioned. But I don't think he is using that scenario. I think Bret said that he had a preamp for the mic and that it was the coupling cable from the output of the preamp to a power amp line input that was in question and that replacement with a very short cable increased the audio level and high frequency response. This doesn't seem very normal to me, that changing from a 15 ft to a 1 ft cable at normal line level medium impedances would make that much difference in frequency response and especially a noticeable gain difference form 20% modulation to 100% or more. A friend of mine is noted for placing resistors and capacitors inside a cable connector for the purpose of EQ or attenuation, rather than modifying the equipment. You have to be careful about using just any old cable in his place; it may have a 1 meg resistor in series with the cable built into the connector end. HIHI. John, WA5BXO -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Jim Bromley Sent: Thursday, January 15, 2004 4:06 AM To: AM Radio List Subject: [AMRadio] Mike Cable Capacitance John Coleman, WA5BXO, posted: (Regarding 50-ohm coax attenuating the high audio frequencies when used as a microphone cable). > I feel there must be some other thing that is overlooked > about the connector or cable. 15 ft of cable would not > normally cause any noticeable effect on audio unless the > terminating Z was greater that a few mega ohms as it would > be for some of the older equipment made for Xtal mike input > but I can't imagine a modern day line input being greater > than 100K ohms The impedance to be concerned with in this case is not the terminating impedance, but rather the source impedance of the microphone itself. In the case of the D-104, although the impedance of the microphone is specified at a nominal 10K-ohms, its actual source impedance is much higher. It is a well-known fact that the element must be terminated in a resistive impedance of at least 10-Megohms to obtain adequate low-frequency response - indicating a source impedance of at least that amount. Additionally, since there is no DC path through a crystal acoustic transducer, one would be suspicious that whatever the actual source impedance is, it would contain a capacitive reactance as a series component. The best solution to the problem is that advocated by several other posters here - incorporate a preamplifier very close to the microphone element (inches, not feet, away) having an imput impedance in the tens of megohms and capable of driving a 500-ohm termination at unity gain. Jim Bromley, K7JEB Glendale, AZ James E. BromleyTel: 623-848-8711 5128 N. 69th Ave. E-Mail: [EMAIL PROTECTED] Glendale, Arizona 85303 ___ AMRadio mailing list AMRadio@mailman.qth.net http://mailman.qth.net/mailman/listinfo/amradio ___ AMRadio mailing list AMRadio@mailman.qth.net http://mailman.qth.net/mailman/listinfo/amradio
RE: [AMRadio] Mike Cable Capacitance
One of us, and it very well could have been me, must have missed something. I though Bret, N2DTS, said that the coupling problem (loss of audio level and high frequency response) was the cable from the preamp to a power amp using line level coupling. I was only mentioning that a high Z input, such as an old style XTAL mic input on the power amp would be a typical problem as you mentioned. But I don't think he is using that scenario. I think Bret said that he had a preamp for the mic and that it was the coupling cable from the output of the preamp to a power amp line input that was in question and that replacement with a very short cable increased the audio level and high frequency response. This doesn't seem very normal to me, that changing from a 15 ft to a 1 ft cable at normal line level medium impedances would make that much difference in frequency response and especially a noticeable gain difference form 20% modulation to 100% or more. A friend of mine is noted for placing resistors and capacitors inside a cable connector for the purpose of EQ or attenuation, rather than modifying the equipment. You have to be careful about using just any old cable in his place; it may have a 1 meg resistor in series with the cable built into the connector end. HIHI. John, WA5BXO -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Jim Bromley Sent: Thursday, January 15, 2004 4:06 AM To: AM Radio List Subject: [AMRadio] Mike Cable Capacitance John Coleman, WA5BXO, posted: (Regarding 50-ohm coax attenuating the high audio frequencies when used as a microphone cable). > I feel there must be some other thing that is overlooked > about the connector or cable. 15 ft of cable would not > normally cause any noticeable effect on audio unless the > terminating Z was greater that a few mega ohms as it would > be for some of the older equipment made for Xtal mike input > but I can't imagine a modern day line input being greater > than 100K ohms The impedance to be concerned with in this case is not the terminating impedance, but rather the source impedance of the microphone itself. In the case of the D-104, although the impedance of the microphone is specified at a nominal 10K-ohms, its actual source impedance is much higher. It is a well-known fact that the element must be terminated in a resistive impedance of at least 10-Megohms to obtain adequate low-frequency response - indicating a source impedance of at least that amount. Additionally, since there is no DC path through a crystal acoustic transducer, one would be suspicious that whatever the actual source impedance is, it would contain a capacitive reactance as a series component. The best solution to the problem is that advocated by several other posters here - incorporate a preamplifier very close to the microphone element (inches, not feet, away) having an imput impedance in the tens of megohms and capable of driving a 500-ohm termination at unity gain. Jim Bromley, K7JEB Glendale, AZ James E. BromleyTel: 623-848-8711 5128 N. 69th Ave. E-Mail: [EMAIL PROTECTED] Glendale, Arizona 85303 ___ AMRadio mailing list AMRadio@mailman.qth.net http://mailman.qth.net/mailman/listinfo/amradio
