[Need Advice] Preamp with variable gain for PaPa Amplifier - diyAudio
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Old 10th June 2015, 04:33 PM   #1
gadut is online now gadut  Indonesia
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Default [Need Advice] Preamp with variable gain for PaPa Amplifier

I have just finished my DAC AD1865, put an order for Alpair 10P, but Amplifier is still on progress to make UMS on 4pcs heatsink, need to find CNC shop.

But i still have a missing link, which i'm not sure it's missing or not. Related to several post on gain matching, my question will be specific to only PaPa amplifier which I only want to build. Below my situation:
- my dac output voltage based on designer calculation should be around 3.6Vrms (I'm still finding a way on how to measure it, i only have dmm)
- amplifier module/parts that is completed : SIT L'Amp and AlephJ (F6, F4 and F5 will be on my next list).

- as far as i know, from above amplifier list, I can separate them into 3 groups
1. High gain amplifier (no need extra gain from preamp) : AlephJ & F6
2. Middle/Low gain amplifier (need extra gain from preamp) : SIT & F5
3. Amplifier without voltage gain : F4

- what I'm thinking for the solution on above 3 groups:
1. Use passive (potentiometer only) or build buffer (my DCB1 build did not succeed, never been able to fix it's shunt)
2. Use middle gain preamp : I already build amb headphone amplifier (α20 powered by σ22) with gain 6
3. Use high gain preamp : i'm thinking of BA-3 FE

Instead of 3 separate preamp, is there any diy preamp design with variable gain? This TKD selector switch will be a nice item to have. I rarely see this variable gain option on diy design, but you can find it in commercial product (which i dont have any interest to buy it)

ps : english is not my native language, hopefully above summary is understandable.
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Old 10th June 2015, 04:40 PM   #2
Zen Mod is offline Zen Mod  Serbia
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there is no so versatile DIY preamp around , marrying two things - wide range of possible gain (and output swing!) and painless switched gain ratio transition - without need to readjust output offset (or at least output node voltage potential)

my is more than capable regarding gain and swing but , as long I'm not implementing offset servo subcircuit , you can't simply switch feedback ratio - to change overall gain
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Old 10th June 2015, 06:20 PM   #3
juma is offline juma  Germany
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Quote:
Originally Posted by gadut View Post
... Instead of 3 separate preamp, is there any diy preamp design with variable gain?...
Sure, I'm testing the prototype now - the complete build with pics will follow.
The input is low-noise, high gm, current production JFET (BF862) cascoded with BJT (if you can remember, it's the "modulated cascode" from F3). This gain cell is loaded with BS170 made gyrator (basically it's a choke with no wire and no iron).
The current through this SE preamp is about 10-12mA, it's set by R4. The gain is set by R16/R10 (6 dB) or R11/R10 (23dB).
The preamp will swing at the output more than 20V_peak (more than needed to drive F4 to full power). Zin is roughly 10k and the Zout is less than 50R (R12 included) so it will drive successfully even F4 without its input buffer.
BC547C has max. Vce of 45V, the BC546B is better choice.
The BJT and BS170 should be equiped with small TO-92 heatsinks (or a piece of copper foil) - they reach about 70 degrees C. I tried different devices - 2sk2013 instead of BS170 is good but with doubled Id, and IRF610 is not good even on cascode duty.
Listening to this pre, it has that nice F3-like "velvety" quality, but with no "darkness" and no impotence - very lovable. Still testing the addition of LATFET SF for a complete amp...
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Last edited by juma; 10th June 2015 at 06:36 PM.
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Old 10th June 2015, 11:00 PM   #4
gadut is online now gadut  Indonesia
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That's impresive Juma, i cant wait for final version, pcb with transistor kit

i have some question:
- any reason why using high voltage supply? Am i correct that all component from +55 until C5 are regulator? What is the voltage on C5?
- instead of only gain 2 and 15 options, what about additional options such as gain 1, 2, 4, 6, 8,15. wouldn't it be nice?
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Old 11th June 2015, 07:05 AM   #5
juma is offline juma  Germany
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Quote:
Originally Posted by gadut View Post
That's impresive Juma, i cant wait for final version, pcb with transistor kit
That won't happen, I don't have enough free time for such an enterprise...


Quote:
Originally Posted by gadut View Post
- any reason why using high voltage supply?
If you want enough output voltage to drive F4 to full power (40V peak-to-peak) it has to come from somewhere. Anyway, 55V is not high voltage - I got it from 2 x 12V AC transformer and voltage doubler...


Quote:
Originally Posted by gadut View Post
Am i correct that all component from +55 until C5 are regulator?
Yes, Q4 and surrounding parts form a capacitance multipler.


Quote:
Originally Posted by gadut View Post
What is the voltage on C5?
About 53V.


Quote:
Originally Posted by gadut View Post
- instead of only gain 2 and 15 options, what about additional options such as gain 1, 2, 4, 6, 8,15. wouldn't it be nice?
Sure, you can do it with 6-position switch where each position will engage different resistor. But I wouldn't do that - it's only 6dB between positions and 6dB is just 2 clicks on the usual 24-position volume attenuator. So high gain/low gain switch makes most sense or you'll be using your volume attenuator in a very narrow range ...

Last edited by juma; 11th June 2015 at 07:09 AM.
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Old 11th June 2015, 10:26 AM   #6
drjbf1 is online now drjbf1  New Zealand
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Maybe for some people the whole point of a true variable gain preamp is that they would not need a volume attenuator at all with the gain switched more discretely. Why throw away any gain you don't need? The closest thing I can think of is the Aleph L, passive then gain.
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Old 11th June 2015, 10:53 AM   #7
juma is offline juma  Germany
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Gadut,
here is an example how you can use your existing PSU to get +55V.
IRF610 needs no heatsink - it dissipates only about 100mW.
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Old 11th June 2015, 03:17 PM   #8
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Juma, thanks for another wonderful design. Looking forward to seeing the
version with a lateral fet follower.

Would 2sk363 be a good part to use if one has difficulty working with SMT components (BF862)?

Thanks,
Dennis
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Old 11th June 2015, 06:15 PM   #9
Hikari1 is offline Hikari1  United States
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Quote:
Originally Posted by gadut View Post
I have just finished my DAC AD1865, put an order for Alpair 10P, but Amplifier is still on progress to make UMS on 4pcs heatsink, need to find CNC shop.

But i still have a missing link, which i'm not sure it's missing or not. Related to several post on gain matching, my question will be specific to only PaPa amplifier which I only want to build. Below my situation:
- my dac output voltage based on designer calculation should be around 3.6Vrms (I'm still finding a way on how to measure it, i only have dmm)
- amplifier module/parts that is completed : SIT L'Amp and AlephJ (F6, F4 and F5 will be on my next list).

- as far as i know, from above amplifier list, I can separate them into 3 groups
1. High gain amplifier (no need extra gain from preamp) : AlephJ & F6
2. Middle/Low gain amplifier (need extra gain from preamp) : SIT & F5
3. Amplifier without voltage gain : F4

- what I'm thinking for the solution on above 3 groups:
1. Use passive (potentiometer only) or build buffer (my DCB1 build did not succeed, never been able to fix it's shunt)
2. Use middle gain preamp : I already build amb headphone amplifier (α20 powered by σ22) with gain 6
3. Use high gain preamp : i'm thinking of BA-3 FE

Instead of 3 separate preamp, is there any diy preamp design with variable gain? This TKD selector switch will be a nice item to have. I rarely see this variable gain option on diy design, but you can find it in commercial product (which i dont have any interest to buy it)

ps : english is not my native language, hopefully above summary is understandable.
While I am sure Juma's preamp would be excellent, I would just put the B1 in front of a BA3 using the pot on the B1 to drive the BA-3 and then the power amp. Some kind of switch to take the BA-3 out of the path for passive use would be needed. I think the BA-3 would work fine as both your "medium" gain and "high" gain preamp. It can be configured for less gain if you desire.

Honestly, I would just use a BA-3 for all. Maybe a switch in the BA-3 to just use the vol pot in front of an amp as a passive solution is all that's needed. I know the F5 doesn't really need a buffer in front of the amp...not sure about the F6 or Aleph.
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Old 11th June 2015, 09:27 PM   #10
juma is offline juma  Germany
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Quote:
Originally Posted by Dennis Hui View Post
... Would 2sk363 be a good part to use if one has difficulty working with SMT components (BF862)? ...
Yes it would - choose one with Idss in 12-15mA range.
It has somewhat higher gm and higher parasitic capacitances, but it's not a deal breaker...
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