[quote author="fum"]There's a second half to the description, that we may or may not need to use in this design, no?[/quote]
No idea. This is Brad's baby. :grin: :guinness:
No idea. This is Brad's baby. :grin: :guinness:
trafoless microphone preamp wish list
- Thread starter bcarso
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chrissugar
Well-known member
[quote author="tk@halmi"]
Interesting that you called out these because they are all using the same design as the Cohen amplifier, and that is not a variety by definition.
OK, so what sort of budget would you consider expensive enough?[/quote]
In my opinion there is enough variation in sound even between those preamps. Anyway, there are not too many ways to make a preamp like that, one is variation on instrumentation amp topology and the other variation on Cohen topology. Or there are others?
If these preamp (AMEK, SSL and co.) can be built for 10-60 euros value in parts per channel, maybe more if exotic components are used, I think for this wire with amplification I would spend up to 150-200 euros for a channel if it is solid state or 200-300 euros in components for one channel if it is tube based.
[quote author="pstamler"]
8) Should be able to accept peak input level of 0dBu or higher without overload at lowest gain setting.
9) In a harmonic distortion test, no harmonics above the fourth should be detectable.
[/quote]
Number 8 was on my list too but I forgot to mention it. Also agree with number 9 which is a good criteria.
chrissugar
Interesting that you called out these because they are all using the same design as the Cohen amplifier, and that is not a variety by definition.
OK, so what sort of budget would you consider expensive enough?[/quote]
In my opinion there is enough variation in sound even between those preamps. Anyway, there are not too many ways to make a preamp like that, one is variation on instrumentation amp topology and the other variation on Cohen topology. Or there are others?
If these preamp (AMEK, SSL and co.) can be built for 10-60 euros value in parts per channel, maybe more if exotic components are used, I think for this wire with amplification I would spend up to 150-200 euros for a channel if it is solid state or 200-300 euros in components for one channel if it is tube based.
[quote author="pstamler"]
8) Should be able to accept peak input level of 0dBu or higher without overload at lowest gain setting.
9) In a harmonic distortion test, no harmonics above the fourth should be detectable.
[/quote]
Number 8 was on my list too but I forgot to mention it. Also agree with number 9 which is a good criteria.
chrissugar
Svart
Well-known member
whew, i think he's going to have his work cut out for him with all these ideas..
as for layout, I suggest a tight SMD layout..
as for layout, I suggest a tight SMD layout..
bcarso
Well-known member
eeeek---I've unleashed a monster (apologies to Hiraga). I started writing the below and there are a zillion posts since. But this one was more or less after the second-to-last of Svart's:
As far as gain, it would be nice to explore Paul Stamler's suggestion and have the first stage gain fixed---this also eliminates the need for reverse log. So the proposed structure is first stage fixed gain, attenuator, second stage fixed gain.
With that in mind though, I would now solicit input for what, within reason, should be our maximum input level. Figure we run up to nearly the rails of the suggested +/- 28V system. Let's give a little room and say +/-25V peak out; maybe internally single-ended, maybe internally balanced. Should we suppose nothing hotter than 100mV rms? That would work out to a numerical gain of about 177 for the first stage.
[added after some look over of the recent posts: maybe it needs an output attentuator if feeding boards that will be easily overloaded]
If the front end of this is doing overall about 1nV/rt Hz at the input, and the second stage is comparable, this would mean that you would start losing significant S/N ratio when the second stage noise gets comparable referred to the first stage. As well you have to consider the noise associated with the attenuator. There are dissipation considerations here as well---if we limit attenuator dissipation to 1/4W the impedance is 1.25k; the worst-case attenuator thermal will be at -6dB and be equivalent to a 313 ohm R or ~2.3nV/rt Hz, so that will dominate the second stage noise but still be totally negligible compared to first stage noise.
But our overall 70dB requires that the second stage have 25dB gain. Assuming a comparable output swing and balanced, this would allow as much as 2V rms swing at the second stage input, which would correspond to 11mV rms at the first-stage input. For the 100mV input the attenuator would have to be set to about -19dB to avoid second-stage output overload. The attenuator noise and second-stage input noise sums rms to 1.75nV/rt Hz while the attenuated first-stage output noise is about 20nV/rt Hz, so we are not degrading the first stage noise significantly.
I?ll have to run these numbers again and check, but it seems as if the 2 stage with intervening attenuator approach would work.
The predicted S/N is decent; neglecting noise in the source: at maximum gain, 1nV/rt Hz at the input (which could get lower if we really work at it---still a bit high compared to the 1.58nV/rt Hz of a 150 ohm resistor, but getting there) becomes 447uV in a 20kHz passband at the output, below 35.6V rms, or about 98dB. At lower gains like the one explored for hot sources the number improves to about 50uV rms out, or about 117dB.
If the mic had thermal noise of 150 ohms the max gain S/N drops to about 93dB. Note that if the preamp were noiseless, with this gain and output swing the best S/N possible would be still only 94dB. And of course real mics and rooms are going to be noisier.
I?m supposing FETs in the front end hence essentially no current noise, and no need for input d.c. blocks unless the mic is phantom-powered. If even lower noise is demanded the easiest way to get there would be with low rbb bipolars. Unfortunately both the FETs and the bipolars considered violate the easy-to-source requirement, although there are some easier-to-get varieties of both that could come within hooting distance.
As far as gain, it would be nice to explore Paul Stamler's suggestion and have the first stage gain fixed---this also eliminates the need for reverse log. So the proposed structure is first stage fixed gain, attenuator, second stage fixed gain.
With that in mind though, I would now solicit input for what, within reason, should be our maximum input level. Figure we run up to nearly the rails of the suggested +/- 28V system. Let's give a little room and say +/-25V peak out; maybe internally single-ended, maybe internally balanced. Should we suppose nothing hotter than 100mV rms? That would work out to a numerical gain of about 177 for the first stage.
[added after some look over of the recent posts: maybe it needs an output attentuator if feeding boards that will be easily overloaded]
If the front end of this is doing overall about 1nV/rt Hz at the input, and the second stage is comparable, this would mean that you would start losing significant S/N ratio when the second stage noise gets comparable referred to the first stage. As well you have to consider the noise associated with the attenuator. There are dissipation considerations here as well---if we limit attenuator dissipation to 1/4W the impedance is 1.25k; the worst-case attenuator thermal will be at -6dB and be equivalent to a 313 ohm R or ~2.3nV/rt Hz, so that will dominate the second stage noise but still be totally negligible compared to first stage noise.
But our overall 70dB requires that the second stage have 25dB gain. Assuming a comparable output swing and balanced, this would allow as much as 2V rms swing at the second stage input, which would correspond to 11mV rms at the first-stage input. For the 100mV input the attenuator would have to be set to about -19dB to avoid second-stage output overload. The attenuator noise and second-stage input noise sums rms to 1.75nV/rt Hz while the attenuated first-stage output noise is about 20nV/rt Hz, so we are not degrading the first stage noise significantly.
I?ll have to run these numbers again and check, but it seems as if the 2 stage with intervening attenuator approach would work.
The predicted S/N is decent; neglecting noise in the source: at maximum gain, 1nV/rt Hz at the input (which could get lower if we really work at it---still a bit high compared to the 1.58nV/rt Hz of a 150 ohm resistor, but getting there) becomes 447uV in a 20kHz passband at the output, below 35.6V rms, or about 98dB. At lower gains like the one explored for hot sources the number improves to about 50uV rms out, or about 117dB.
If the mic had thermal noise of 150 ohms the max gain S/N drops to about 93dB. Note that if the preamp were noiseless, with this gain and output swing the best S/N possible would be still only 94dB. And of course real mics and rooms are going to be noisier.
I?m supposing FETs in the front end hence essentially no current noise, and no need for input d.c. blocks unless the mic is phantom-powered. If even lower noise is demanded the easiest way to get there would be with low rbb bipolars. Unfortunately both the FETs and the bipolars considered violate the easy-to-source requirement, although there are some easier-to-get varieties of both that could come within hooting distance.
bcarso
Well-known member
I see now (since I wrote the last) that 0dBu is suggested as the input overload threshold. That is tough, but it would seem from the preceding that the first stage gain could be still lower and preserve the basic structure.
As mentioned earlier the option as well is to ensure the same harmonic structure while doing variable first-stage gain---and that would be something new as far as I know.
Paul S., give me a number for fifth and above please ;-). If you hunt hard enough there's going to be some---it's like going in for a drug test after being downwind at a concert. But I agree it is a laudable goal to make the higher-order truly negligible.
I also agree that this is probably going to evolve to a moderate-cost or highish-cost design, as otherwise what is the point to doing as Chris indicates just another trafoless preamp? Maybe there will be a trickle-down effect.
As mentioned earlier the option as well is to ensure the same harmonic structure while doing variable first-stage gain---and that would be something new as far as I know.
Paul S., give me a number for fifth and above please ;-). If you hunt hard enough there's going to be some---it's like going in for a drug test after being downwind at a concert. But I agree it is a laudable goal to make the higher-order truly negligible.
I also agree that this is probably going to evolve to a moderate-cost or highish-cost design, as otherwise what is the point to doing as Chris indicates just another trafoless preamp? Maybe there will be a trickle-down effect.
featherpillow
Well-known member
This is such a great thread--and such a wonderful combination of ideas, minds and personalities.
Hey Brad, what are you thinking about in terms of topology? Anything similar to some of the circuit ideas you developed recently in light of the Darlington thread? Any Diamond-buffer-based ideas?
Like most people, I've read through the Hiraga stuff, and it certainly sounds like a wonderful excercise. While the cloning of designs is fun, it seems a little limited to me in terms of a learning experience. It's this type of thing that I love engaging in and learning about.
Hey Brad, what are you thinking about in terms of topology? Anything similar to some of the circuit ideas you developed recently in light of the Darlington thread? Any Diamond-buffer-based ideas?
Like most people, I've read through the Hiraga stuff, and it certainly sounds like a wonderful excercise. While the cloning of designs is fun, it seems a little limited to me in terms of a learning experience. It's this type of thing that I love engaging in and learning about.
bcarso
Well-known member
Heh. I'm still in the information-gathering mode at the moment. What is that great dialogue in Blazing Saddles when Hedley Lamarr is cranking away: "My mind is aglow with transient nodes of thought---careening through a cosmic vapor of invention." Taggart: "Ditto!" Lamarr: "Ditto!! Ditto!! What do you mean 'Ditto', you provincial putz?!"
Svart
Well-known member
sk170s are now being manufactured by Linear technologies... doubt we will have a hard time finding these..
bcarso
Well-known member
True. Actually Toshiba still makes them as well, just not the ~dual equivalent, the SK389. But as we know Toshiba does not consider many places in the world to be worth supporting for their better parts.
Svart
Well-known member
yeah some places still have stock of the sk389.. I stocked up on them a while ago and have a handful lying around but I am considering ordering a larger supply before they dry up completely...
featherpillow
Well-known member
I think its Linear Systems, isn't it?
http://www.linearsystems.com/products.html#GlossE
chrissugar
Well-known member
2SK170 and 2SJ74 is still manufactured, only the 2SK389 and 2SJ109 duals are not made anymore. I suppose if we will have something based on these two jfets I think we can make a huge group buy.
chrissugar
chrissugar
chrissugar
Well-known member
I don't know if it is a good idea, but to preserve the sound at any gain I think it would be a solution to use for the first stage and the second stage, identical amplification blocks with approximately 30-35 dB gain each.
For high levels we can use only the first stage with some kind of attenuation to achieve 0 to 30dB gain and for gains over 30dB we can add the second block.
For 0-30dB amplification the output of the preamp would be the first gain block out, for 30-60dB the preamp output would be switched to the out of the second gain block. In this case both blocks should be identical, and have a buffer for low Z driving capability, and should have very low distortion to not change significantly the sound depending on the fact that there is one gain block or two in the circuit.
Maybe someone can confirm that the change in sound between one or two low distortion gain blocks is smaller then the change in sound at different gain settings in a preamp where the gain is changed at the feedback circuit.
chrissugar
For high levels we can use only the first stage with some kind of attenuation to achieve 0 to 30dB gain and for gains over 30dB we can add the second block.
For 0-30dB amplification the output of the preamp would be the first gain block out, for 30-60dB the preamp output would be switched to the out of the second gain block. In this case both blocks should be identical, and have a buffer for low Z driving capability, and should have very low distortion to not change significantly the sound depending on the fact that there is one gain block or two in the circuit.
Maybe someone can confirm that the change in sound between one or two low distortion gain blocks is smaller then the change in sound at different gain settings in a preamp where the gain is changed at the feedback circuit.
chrissugar
bcarso
Well-known member
We could just make the 0dBu input requirement with a balanced output of the +35dB first stage and a little over 30V rails---brutal but achievable.
I have as I've mentioned a large number of the 2SK170BL equivalent, the part characterized for switching apps but the same chip inside, the 2SK364BL. Also I have a lot of 2SK389V's and about 200 2SJ109BL's. I don't anticipate getting into production with any of these so they are ~available.
But Linear Systems said they were doing a dual soon as well, at least for the N variety, and claimed it would be even lower noise than the 389.
If you want to use the individual chips, matching requires a large number and a tedious sort. But there is the advantage of not having the substrate capacitance to deal with as you have with the duals.
I have as I've mentioned a large number of the 2SK170BL equivalent, the part characterized for switching apps but the same chip inside, the 2SK364BL. Also I have a lot of 2SK389V's and about 200 2SJ109BL's. I don't anticipate getting into production with any of these so they are ~available.
But Linear Systems said they were doing a dual soon as well, at least for the N variety, and claimed it would be even lower noise than the 389.
If you want to use the individual chips, matching requires a large number and a tedious sort. But there is the advantage of not having the substrate capacitance to deal with as you have with the duals.
bcarso
Well-known member
[quote author="chrissugar"]
Maybe someone can confirm that the change in sound between one or two low distortion gain blocks is smaller then the change in sound at different gain settings in a preamp where the gain is changed at the feedback circuit.
chrissugar[/quote]
As Paul Stamler said earlier in this thread, he recently checked out a preamp with entirely different harmonic structure at different feedback-determined gains.
Maybe someone can confirm that the change in sound between one or two low distortion gain blocks is smaller then the change in sound at different gain settings in a preamp where the gain is changed at the feedback circuit.
chrissugar[/quote]
As Paul Stamler said earlier in this thread, he recently checked out a preamp with entirely different harmonic structure at different feedback-determined gains.
chrissugar
Well-known member
[quote author="bcarso"]
As Paul said earlier in this thread, he recently checked out a preamp with entirely different harmonic structure at different feedback-determined gains.[/quote]
Yes, I read that, and from my own experience I can say that I hear the change in sound from low gain and high gain in API topology type preamp.
My question is more like, is the diference in sound between one gain block and two blocks in series comparable with the change in sound in a variable feedback circuit. I suppose two optimised low distortion gain blocks will produce less change then the variable feedback one.
chrissugar
As Paul said earlier in this thread, he recently checked out a preamp with entirely different harmonic structure at different feedback-determined gains.[/quote]
Yes, I read that, and from my own experience I can say that I hear the change in sound from low gain and high gain in API topology type preamp.
My question is more like, is the diference in sound between one gain block and two blocks in series comparable with the change in sound in a variable feedback circuit. I suppose two optimised low distortion gain blocks will produce less change then the variable feedback one.
chrissugar
It is interesting to note that Audiophiles never do variable gain on phono stages citing what Paul has mentioned.
On the other hand they are not challanged with the noise accumulation over multiple track that plagues studio recording.
I wonder if three switchable stages, with fixed 20dB gain each, would have too much distortion at the end.
Fine volume control is another hurdle if all the stages are balanced unless you can use something like a multi-deck 24 position attenuator.
Tamas
On the other hand they are not challanged with the noise accumulation over multiple track that plagues studio recording.
I wonder if three switchable stages, with fixed 20dB gain each, would have too much distortion at the end.
Fine volume control is another hurdle if all the stages are balanced unless you can use something like a multi-deck 24 position attenuator.
Tamas
chrissugar
Well-known member
Tamas,
I supose three blocks would be too complicated, more distortion, and probably the only benefit would be a bit lower noise.
If the preamp will be ballanced from front to end probably some sort of balanced stepped attenuator will be the right solution.
chrissugar
I supose three blocks would be too complicated, more distortion, and probably the only benefit would be a bit lower noise.
If the preamp will be ballanced from front to end probably some sort of balanced stepped attenuator will be the right solution.
chrissugar
bcarso
Well-known member
Yes, unfortunately nonlinearities have a way of multiplying in cascades whether via feedback or not. Even-orders are relatively amenable to cancellation whehter with balanced or carefully cascaded alternating-polarity designs, but odd-orders are much more troublesome.
Hi guys, I just popped in. Yet another highly intersting thread!
One thing I'd like to discuss is input impedance. I see that many here want variable input impedance. Having used some preamps with variable input impedance such as A*R*T and GT Vi*Pre, I have mixed feelings about variable impedance. The inexpensive A*R*T preamp for instance had bad noise performance with my t*bone RB-500 Ribbon, and I suspect it had to do with the VI feature. The Vi*Pre is another animal, of course. Yet, to be honest, I had little use for its VI switch. So my question is:is there anyone who has had great experiences with VI or do we want VI just because it's en vogue?
In my experience VI is overrated. When you make up the level differences due to varied impedance, sound differences are negligible in most cases.
Another cool possibility IMHO would be to have a pretty high fixed input impedance. If impedance was about 10k, we could use smaller input caps and would thus be able to use e.g. polyprop at relatively low cost. How's that?
One thing I'd like to discuss is input impedance. I see that many here want variable input impedance. Having used some preamps with variable input impedance such as A*R*T and GT Vi*Pre, I have mixed feelings about variable impedance. The inexpensive A*R*T preamp for instance had bad noise performance with my t*bone RB-500 Ribbon, and I suspect it had to do with the VI feature. The Vi*Pre is another animal, of course. Yet, to be honest, I had little use for its VI switch. So my question is:is there anyone who has had great experiences with VI or do we want VI just because it's en vogue?
In my experience VI is overrated. When you make up the level differences due to varied impedance, sound differences are negligible in most cases.
Another cool possibility IMHO would be to have a pretty high fixed input impedance. If impedance was about 10k, we could use smaller input caps and would thus be able to use e.g. polyprop at relatively low cost. How's that?
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