Svart
Well-known member
you've fooled us!
:green:
Sorry couldn't resist!
:green:
Sorry couldn't resist!
Mic Preamp Schematic Collection
- Thread starter Samuel Groner
- Start date
Help Support GroupDIY Audio Forum:
bcarso
Well-known member
:green:
Hey , first of all thanks for this forum everybody ! Samuel I builded your design A with a few changes. This is what I did:
C4,C5 polyester caps to 2,2 uF.
R7 changed to 200k.
Two extra input Q, one per side, I used 2SA970
R18,R19 to 1k5
R16 to 11k5
U1 to OPA 2604
U2 to sep. OPA 604
I also kept the output-stage at unity gain all res. 10k
The output offset (bias)was 262/274 mV at the first stage ampl.outputs
and at the second output there was 22mV between R44 and R45.
I tried to keep the 3 Q;s beta per side as neer as possible.
( beta 475 and 478 in those samples I took out.)
How did it measure; very well indeed ! the upper rolloff was about -6 at 200 kHz ,this with 10k//120 pf at the output stage. A smooth rounded squarevawe and no oscillations, at any gain from 6 to 60 dB.
How did it sound. Swell !! or to say quiet and clean with a big headroom.
Thanks and cheers, says Bo
C4,C5 polyester caps to 2,2 uF.
R7 changed to 200k.
Two extra input Q, one per side, I used 2SA970
R18,R19 to 1k5
R16 to 11k5
U1 to OPA 2604
U2 to sep. OPA 604
I also kept the output-stage at unity gain all res. 10k
The output offset (bias)was 262/274 mV at the first stage ampl.outputs
and at the second output there was 22mV between R44 and R45.
I tried to keep the 3 Q;s beta per side as neer as possible.
( beta 475 and 478 in those samples I took out.)
How did it measure; very well indeed ! the upper rolloff was about -6 at 200 kHz ,this with 10k//120 pf at the output stage. A smooth rounded squarevawe and no oscillations, at any gain from 6 to 60 dB.
How did it sound. Swell !! or to say quiet and clean with a big headroom.
Thanks and cheers, says Bo
Samuel Groner
Well-known member
Thanks for reporting back, glad you like the result!
Samuel
Samuel
Animatic
Well-known member
[quote author="Wavebourn"][quote author="JohnRoberts"]
Not my fault mon, if you’re plugging into a mic input, there’s a very good chance you’ll see phantom voltage sometime in your product life.[/quote]
Especially during a live performance when everybody are under stress.
[quote author="mediatechnology"]
I've questioned a lot of engineers about this gain switch/pot thing and the consensus so far has been a precise stepped switch with a post-pre gain trim pot. [/quote]
I am both developer and end user and second that.[/quote]
I have been wading along through this thread gleaning lots of info.
But sadly Mr Roberts and Mr Groner leave me swirling in their dust.
I am none the less learning things, and hopefully can make them
applicable to my project needs.
I have to agree with John if somebody built a radio mic receiver and
didn't take phantom power into consideration it's their fault,
especially taking Wavebourn's scenario into consideration.
Normal use environment can not be expected to be dynamic mics only,
nor can it be 100% predictable in live situatuions that you can
SEE for SURE that the phantom is off for a given channel.
Now you get a ribbon mic and connect it without checking for phantom, more fool you.
I am wanting to build a dedicated preamp for a Shinybox ribbon
and not have to worry about this issue at all. But still have
optimized gain/ I/O structure for this particular mic's issues.
I have looked at head and tail here,
and am working through the middle now. (yikes)
Has anyone built these designs from page one recently
or is this still mostly theoretical discussion?
Not my fault mon, if you’re plugging into a mic input, there’s a very good chance you’ll see phantom voltage sometime in your product life.[/quote]
Especially during a live performance when everybody are under stress.
[quote author="mediatechnology"]
I've questioned a lot of engineers about this gain switch/pot thing and the consensus so far has been a precise stepped switch with a post-pre gain trim pot. [/quote]
I am both developer and end user and second that.[/quote]
I have been wading along through this thread gleaning lots of info.
But sadly Mr Roberts and Mr Groner leave me swirling in their dust.
I am none the less learning things, and hopefully can make them
applicable to my project needs.
I have to agree with John if somebody built a radio mic receiver and
didn't take phantom power into consideration it's their fault,
especially taking Wavebourn's scenario into consideration.
Normal use environment can not be expected to be dynamic mics only,
nor can it be 100% predictable in live situatuions that you can
SEE for SURE that the phantom is off for a given channel.
Now you get a ribbon mic and connect it without checking for phantom, more fool you.
I am wanting to build a dedicated preamp for a Shinybox ribbon
and not have to worry about this issue at all. But still have
optimized gain/ I/O structure for this particular mic's issues.
I have looked at head and tail here,
and am working through the middle now. (yikes)
Has anyone built these designs from page one recently
or is this still mostly theoretical discussion?
Samuel Groner
Well-known member
Do you have a specific design in mind?
Personally I've only implemented design A and B, with some slight changes (which are not a must). I'd expect the other designs to work without much troubles though.
Samuel
Personally I've only implemented design A and B, with some slight changes (which are not a must). I'd expect the other designs to work without much troubles though.
Samuel
Animatic
Well-known member
Hi Samuel
Well I have been looking at the NYD Mila, pretty clean and straight forward.
I like putting tubes before digital. I like class A in general, of you have the band width no issues.
But also your G ri pre amp looks pretty straightforward.
Less in the path often means clean sound, if there isn't
strong pushing and pulling from outside the circuit to change it.
If I had a PCB schematic/layout or mask
I can find someone how can actually make the board in Bangkok.
But I don't have the programs to actually layout a design.
I'd do 4-8 of them. Build two and do some room recording to test.
(Looking for 36 preamps in all, JUST for me.)
Would this be ok with a fairly long line after it? Say 30meters?
And the C ri for ribbon mics I want 2 of those, maybe 4.
A much more convoluted unit, but ribbons seem to need special care.
I want a matched pair of ribbons and preamps for room recording
a warm retro sound for acoustic music.
In all case I want to control the preamps gain from a distance.
Shortest mic to preamp run, then line level to monitoring station.
But remote gain control and at least an LED monitoring section back
in the CR for level ref.
Well I have been looking at the NYD Mila, pretty clean and straight forward.
I like putting tubes before digital. I like class A in general, of you have the band width no issues.
But also your G ri pre amp looks pretty straightforward.
Less in the path often means clean sound, if there isn't
strong pushing and pulling from outside the circuit to change it.
If I had a PCB schematic/layout or mask
I can find someone how can actually make the board in Bangkok.
But I don't have the programs to actually layout a design.
I'd do 4-8 of them. Build two and do some room recording to test.
(Looking for 36 preamps in all, JUST for me.)
Would this be ok with a fairly long line after it? Say 30meters?
And the C ri for ribbon mics I want 2 of those, maybe 4.
A much more convoluted unit, but ribbons seem to need special care.
I want a matched pair of ribbons and preamps for room recording
a warm retro sound for acoustic music.
In all case I want to control the preamps gain from a distance.
Shortest mic to preamp run, then line level to monitoring station.
But remote gain control and at least an LED monitoring section back
in the CR for level ref.
Samuel Groner
Well-known member
No tubes here... Can't offer any help for the PCB layout either (spare time is very rare), but there is free layout software available I think?
Samuel
Samuel
Animatic
Well-known member
Figured as much.
Oh joy another learning curve,
on another OS, even worse.
Oh joy another learning curve,
on another OS, even worse.
skipwave
Well-known member
Would the CM filters described in the following datasheet be suitable for the transformerless designs?
http://www.coilcraft.com/pdfs/trf.pdf
Thanks!
http://www.coilcraft.com/pdfs/trf.pdf
Thanks!
jdbakker
Well-known member
Hi Samuel,
Quick Q: I've been working on a pre similar to your Design C (gain distributed over two stages). Is there any particular reason why you have the diff amp stage after the second gain stage? I was thinking about swapping them, in order to eliminate the CM part of the signal as soon as possible, to reduce the impact of unintended CM->DM conversion in the second gain stage. I cannot theorize or sim a reason not to do this, but all two-stage designs that I've seen (including yours and the SSL9k) have the differential stage last.
Am I missing something here?
Thanks,
JDB.
[I can see resistor noise being a factor, but that's fixable with lower values. CMRR trimming may get harder, but there are ways around that too]
Quick Q: I've been working on a pre similar to your Design C (gain distributed over two stages). Is there any particular reason why you have the diff amp stage after the second gain stage? I was thinking about swapping them, in order to eliminate the CM part of the signal as soon as possible, to reduce the impact of unintended CM->DM conversion in the second gain stage. I cannot theorize or sim a reason not to do this, but all two-stage designs that I've seen (including yours and the SSL9k) have the differential stage last.
Am I missing something here?
Thanks,
JDB.
[I can see resistor noise being a factor, but that's fixable with lower values. CMRR trimming may get harder, but there are ways around that too]
Samuel Groner
Well-known member
Basically yes, note however that the inductance is much lower than the value I recommended which makes the filter somewhat less efficient at least at lower frequencies. You might want to quickly run a simulation to check for roughly flat frequency response with the new value.
The CMRR of the overall mic pre will be (as a first-order approximation) improved by the differential gain of the additional gain stage as this gain stage has unity gain for CM signals; you cannot restore that advantage with trimming. If the CMRR of the differential amplifier is limited to say 80 dB (trimmed or untrimmed does not matter) the overall CMRR of the mic pre will be 140 dB at 60 dB gain and the configuration as shown (of course there's always a second-order effect which limits things but they tend to be benign IME at least mid-band). If you re-arrange the topology the overall CMRR will be reduced by the gain of the second (or rather now third) stage. Check the CMRR curves of those IC instrumentation amps, they nicely show the gain-dependent CMRR.
And there will typically be a noise advantage at least at medium and low gains as you noted. Hard to get such low feedback impedance values in the differential amp as one can with the noninverting configuration (220 Ohm for my design C at maximum gain).
Currently I'm working on solutions to keep bandwidth/distortion approximately constant with gain even for a single stage topology by reducing the current-feedback inverting input impedance. Things look promising, too early though to show results.
Samuel
sahib
Well-known member
[quote author="JohnRoberts"]
The input impedance of the transistor stage without feedback would just be emitter load divided by Hfe, .....
JR[/quote]
Hi John,
I am just catching up with the old posts and I apologise in advance if I missed anything. I think this should have been emitter load multiplied by Hfe since you are looking into the transistor from the base circuit.
The input impedance of the transistor stage without feedback would just be emitter load divided by Hfe, .....
JR[/quote]
Hi John,
I am just catching up with the old posts and I apologise in advance if I missed anything. I think this should have been emitter load multiplied by Hfe since you are looking into the transistor from the base circuit.
yes
JR
JR
jcharles00
Well-known member
sorry for the necropost, but i notice the pdf's in the initial posting no longer work. any chance they could be re-posted?
jdbakker
Well-known member
jcharles00 said:
That happened in the transition to the new forum software. You can fix them by hand by copying the links and removing the string "www.groupdiy.com/" from them. For example:
http://www.groupdiy.com/www.sg-acoustics.ch/analogue_audio/microphone_preamplifiers/pdf/A_r1.pdf
should be
http://www.sg-acoustics.ch/analogue_audio/microphone_preamplifiers/pdf/A_r1.pdf
HTH,
JDB.
+1
beat me to it...
PDFs are still there, but links corrupted.
JR
beat me to it...
PDFs are still there, but links corrupted.
JR
jcharles00
Well-known member
jdbakker said:
ahh. excellent! thanks for the info!
I may have missed it, but should this info be in a sticky somewhere?
JohnRoberts said:
Yes those PN sound familiar, I think they changed to those at my old gig when ROHM parts went obsolete (happened after I left).. The 737s that I still have a few of in the back room are also 2 ohm Rbb and on paper a red C hair less noise .55nV/rt Hz... but into 150 or 200 ohms no worries...both parts are much better than needed.
Yes I recall the 4403 in M&F ... but keep in mind when that book was written ('70s). The supposedly "low noise" transistors like 5088? or something like that were a joke into lower impedances... IMO Buff was wise to look at 500mA med power devices and screen for 1/F.
At least the Japanese were fanatical enough about their Hi-fi to develop devices for head amps. As I recall ROHM bought the small company that originally tooled the 737/786 parts.
I called and spoke with Fitchen who was a Prof. at University of Bridgeport as I lived a couple of towns away at the time. I wanted to pick his brain about low noise design beyond what was in his book and he ended up asking me if I wanted to contribute a chapter for his next book...
I was flattered but wrong answer. I was more like an empty sponge than a font of wisdom. I'm self taught so writing for a college level engineering text is the last thing I needed to be doing, especially 25-30 years ago when I was but a mere pup... Maybe he was just trying to get rid of me.. It worked.JR
[/quote]
JR
I am planing to mod some old Langevin cards and I would like to get a better transistors for these mic preamps. These card using the 2N3391 (Q1 in the picture) and 2N3417 (Q2 in the picture). Could you give me a advise in order to get better transistors in order to get a better noise figure and performance????......
I will wait for your comments, thanks
opacheco.
Attachments
You might want to experiment with a few parts and see what you get.
It looks like the 3391A may be a low noise part (or lowest of that series) The 3390 is higher beta tho..But specifying NF at a few hundred ohms and using it at tens of K makes NF academic. They are a 500mA part (good for low Rbb) but clearly a lower current part could be used.
From looking at the impedances around that circuit I would suggest a decent low noise JFET (for at least Q1, but you would need to revisit the DC biasing scheme.
JR
It looks like the 3391A may be a low noise part (or lowest of that series) The 3390 is higher beta tho..But specifying NF at a few hundred ohms and using it at tens of K makes NF academic. They are a 500mA part (good for low Rbb) but clearly a lower current part could be used.
From looking at the impedances around that circuit I would suggest a decent low noise JFET (for at least Q1, but you would need to revisit the DC biasing scheme.
JR
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