reduce gain of discrete pre w. schematic
Old 4th April 2012
  #1
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reduce gain of discrete pre w. schematic

Hello,
I have attached the schematic from the Tascam M600 Service Manual.
Gain of the amp can be varied with R120 from about 28 to 72. When recording drums i have to enable the 30db pad on almost every channel. In my understanding that is a waste of s/n ratio.
I have no real knowledge about discrete circuit design. What i would like to know:
Can i adjust the circuit to have lower gain? 0 to say 50 would be nice. I don´t think i need more than that, especially not from this pre as it gets noisy when cranked up high.
What should be changed, and are there any drawbacks?
Transistors are 2SC-1844F
Datasheet: 2SC1844 datasheet(1/3 Pages) NEC | NPN SILICON TRANSISTOR

Oh, and one more thing, not the intention of this thread but just a side question:
if i replace the caps, i always read about the low ESR electrolyts. Panasonic FC and others. But what about the Bipolar Caps C111 C112 in this schematic? If someone has a suggestion how to deal with those i would be thankful.
Attached Thumbnails
reduce gain of discrete pre w. schematic-m600-micpre.jpg  
Old 4th April 2012
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From the section of schematic showing, the minimum gain looks a lot lower than 28, unless there's another gain stage not shown.

The circuit topology doesn't let you remove a lot more gain from that stage, since the input devices have to be limited to only a few volts of signal swing around 0V.

using a pad is not really a huge problem. If you want, you could make the pad a little less severe.

c111/c112 are electrolytic too and would benefit from replacing if old and tired.

JR
Old 4th April 2012
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Thanks!
There is a diagram in the manual showing the gains/levels through the console.
MIC In is supposed to be within -68 and -26 without pad - to be brought to -8 when it enters the eq.
So thanks, you were right, gain from that stage is actually 18-60 if i got it right this time.
Would you mind telling me how you could estimate the gain from this schematic?

Quote:
since the input devices have to be limited to only a few volts of signal swing around 0V
What do you mean by input device? input signal?

C111/112 are labled bipolar. Can i replace them with regular Caps? If so, does it mater how i orientate them?

Thanks.
Old 5th April 2012
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Quote:
Originally Posted by jrp View Post
Thanks!
There is a diagram in the manual showing the gains/levels through the console.
MIC In is supposed to be within -68 and -26 without pad - to be brought to -8 when it enters the eq.
So thanks, you were right, gain from that stage is actually 18-60 if i got it right this time.
Would you mind telling me how you could estimate the gain from this schematic?
From looking at the component values?

1+ ((13k+13K)/(5k+22)) so roughly 6x but what do I know?
Quote:
What do you mean by input device? input signal?
The input devices... Q101 and Q102

Their collectors are sitting around +7.5V, so you don't want the input audio signal to to exceed that.
Quote:
C111/112 are labled bipolar. Can i replace them with regular Caps? If so, does it mater how i orientate them?

Thanks.
regular non-polar caps would be nice... perhaps standard polar caps oriented based on actual DC offset, or perhaps two polars in series + to +, but they'd need to be 2x the capacitance.

JR
Old 5th April 2012
  #5
If you change R108 to 130 ohms, that will make the pad do -15 db. R118 and R119 set minimum gain, you can maybe cut that 1/2 to 6k or so, that will make minimum gain -6 db less than it is. Check it on a scope as less minimum gain may make the circuit unstable. If you see any oscillations, try installing a 150 pf cap in the empty holes for C110.

If you use polarized caps, check the DC on each leg first. Mount them with the more positive reading to the + pin on the cap. If you double the values, you will get more low end.

To lower noise, change the 2SC1844's to 2SC2545 Hitachi or 2SC3329BL Toshiba or Renesas.
Old 5th April 2012
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Thank you!
In case there is no dc around the bip. cap:
So i understand that modern electrolyts don´t mind getting a little bit of reversed polarization?
If the signal swings around 0V and the cap is connected to a point being at ground potential, it will see a "wrong" polarity at every half wave, right? But this can be tolerated, right?

As For the noise, it is surprisingly quiet. With 200 Ohms source I really have to turn it way up to get a reading on my soundcard. No hum also.
To me it doesn´t sound too bad already.
But a matched transistor pair might have other benefits?

I may check out one channel and compare it against an INA217 preamp thats on my bench right now. If it can compete with no or little changes then it would be easiest for me to change more channels. I just doubt that it can... Sound is ok but nothing special.

Quote:
From looking at the component values?

1+ ((13k+13K)/(5k+22)) so roughly 6x but what do I know?
Ok, i can find the corresponding resistors. I just had no idea how gain is set in a circuit like this. All i know (a little) is opamps and cmos...
Discrete circuits seem so much more complicated...

Quote:
Their collectors are sitting around +7.5V, so you don't want the input audio signal to to exceed that.
isn´t that actually very much voltage for an input signal?
Old 5th April 2012
  #7
El caps can tolerate 20% reverse bias and still work. I wouldn't do that though. The output el cap will see a fixed DC voltage. Just orient the polarized cap properly with the + pin towards the + voltage, or the - pin towards the - voltage.

The input signals from a microphone will not get close to 7.5 volts. For the large 470 uf gain set cap, I would use a polarized 1000 uf cap, it sees the same DC on both sides. If you are worried about bias reversal, install a 1N4148 diode across that cap, cathode towards the - pin.

Matching the front end transistor pair hfe on a meter can improve the CMRR specs of the preamp front end.

Use polarized caps on the front end that block 48 volt phantom. You can raise the input impedance if you remove the 2.4k resistor across the inputs. I would replace the 22k input loading resistors with 10k ohms metal film, that also may lower noise a bit. Match them on a meter to improve CMRR.
Old 5th April 2012
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Thanks for those hints. I´ll report back if i have done a channel. Hope i get to it soon, first i´ll finish those INA217 pres...
I have seen some comments from you on this desc in this forum, so you seem to have some experience with it. Considering the amount of work and the results to be expected, do you think it is worth the effort?
INA217 with dc-servo+DRV134 make such a simple build, maybe i should focus on those... I already got the chips, a case and psu. Still it involves more work than changing some parts on an old console...
My goals are good clean sound, quick and easy build. I need 16-20 pres.
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