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Old 29th March 2011, 02:48 PM   #1
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Question Tone control for my diy preamp

I have built a DOZ Preamp (Minimalist Discrete Hi-Fi Preamp) and I absolutely love the sound. Only, I used 3904/3906 for the transistors, although all I hear through speakers when silence is just that, silence. I was expecting there to be slight noise since I have not used the low noise transistors.

I have been using my DIY LM3886 stereo amplifier for over a year, with the preamp a circuit consisting of TLE2082 and BUF634T in feedback paths. I like the sound, but not so much as this preamp. I will be completely removing that preamp from my amp case, along with volume control, etc.

I will be building a separate case for this DOZ preamp, with volume control separate for each channel and I want to add tone control as well (with a defeat switch). I also need a mono (summed) output as well, for the powered subwoofer.

So, I don't have much experience with DIY Audio. Where along the circuit do you suggest I have the tone control circuit, and volume controls? Should the tone control be the last stage? The first stage? I'm assuming the best place to take the signals for summing to mono out would be at the end of the entire signal path. I'm really not sure here. I'm not even sure how to incorporate the defeat. Horrible isn't it?

Thanks for any and all education. I appreciate it.

Last edited by odysseybmx414; 29th March 2011 at 02:50 PM.
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Old 29th March 2011, 04:16 PM   #2
lineup is offline lineup  Sweden
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Old 29th March 2011, 05:19 PM   #3
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Originally Posted by lineup View Post
I could replace U1A(and B) with the DOZ, correct? (http://sound.westhost.com/p97-f1.gif) And for U2A,B I can use OPA2134 and bias the op amps into class A. I will not use a balance pot, but will use separate single pots for individual volume control for each channel.

This is what I am thinking:

DOZ------------POT--------TO R111/R211 FROM (http://sound.westhost.com/p97-f1.gif) ---OUT



Though, I suppose I may want to keep U1A,B for the gain. I could invert it as well to correct the inverted signal coming from the tone control circuit, correct?

I assume I can also omit the third stage (http://sound.westhost.com/p97-f4.gif)

Or is there a different circuit perhaps using only a few trasistors instead of many op amps?

I could always get this: STEREO TONE CONTROL AMPLIFIER KIT - BASED ON NAD 3020! - eBay (item 380172022891 end time Apr-20-11 01:47:33 PDT)

but then that would defeat my entire purpose of doing this.

Thanks again.
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Old 29th March 2011, 05:20 PM   #4
marusmk is offline marusmk  Poland
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Quote:
Originally Posted by lineup View Post
PCB files (Sprint-Layout format):

forum.cxem.net/index.php?showtopic=11829&view=findpost&p=504338

odysseybmx414 - you have a PCB for DOZ?
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Old 29th March 2011, 05:26 PM   #5
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Quote:
Originally Posted by marusmk View Post
PCB files (Sprint-Layout format):

forum.cxem.net/index.php?showtopic=11829&view=findpost&p=504338

odysseybmx414 - you have a PCB for DOZ?
Did not make a pcb, I soldered mine onto proto board.
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Old 30th March 2011, 04:57 PM   #6
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Quote:
Originally Posted by odysseybmx414 View Post
I could replace U1A(and B) with the DOZ, correct? (http://sound.westhost.com/p97-f1.gif) And for U2A,B I can use OPA2134 and bias the op amps into class A. I will not use a balance pot, but will use separate single pots for individual volume control for each channel.

This is what I am thinking:

DOZ------------POT--------TO R111/R211 FROM (http://sound.westhost.com/p97-f1.gif) ---OUT



Though, I suppose I may want to keep U1A,B for the gain. I could invert it as well to correct the inverted signal coming from the tone control circuit, correct?

I assume I can also omit the third stage (http://sound.westhost.com/p97-f4.gif)

Or is there a different circuit perhaps using only a few trasistors instead of many op amps?

I could always get this: STEREO TONE CONTROL AMPLIFIER KIT - BASED ON NAD 3020! - eBay (item 380172022891 end time Apr-20-11 01:47:33 PDT)

but then that would defeat my entire purpose of doing this.

Thanks again.
Anybody? I appreciate it.
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Old 30th March 2011, 05:55 PM   #7
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"I could replace U1A(and B) with the DOZ, correct?"


U1 gives a nominal 6 db of gain, should be OK with less gain.



"And for U2A,B I can use OPA2134 and bias the op amps into class A."

I use the 2134 in my preamp in which I use Rod's P97 boards in a dual mono mode.Two separate boards that each do one channel. Reason? Channel separation. With one board and both channels on it I only got 35 db of separation at 1000 hz. Not good.

Never tried to bias into class A, may try it some day.


"I will not use a balance pot, but will use separate single pots for individual volume control for each channel."


Better rethink that move, you need the balance pot to keep the gain structure the same.I used a dual 50K pot, linear, to separate the channels and get a final separation figure of 90 db tested from 20 to 20,000 hz. That's a very good figure. Usually it gets worse as the frequency rises.

I used a 100K volume control from Goldpoint. Pricey but it is definitely worth the cost. Channel tracking so close I can't measure the difference. Separate pots will work fine though so go for it.




"Though, I suppose I may want to keep U1A,B for the gain. I could invert it as well to correct the inverted signal coming from the tone control circuit, correct?"


No, Rod's circuit is already set up to correct the phase swap caused by the tone control circuit.


"I assume I can also omit the third stage "


Nope again, tone control and bal/vol stage need a buffer to work correctly.


"Or is there a different circuit perhaps using only a few transistors instead of many op amps?"


There are only three in the complete amp. One channel of course. Why don't you try using the circuit stock as shown on the website? You might like it. In the tone bypass mode, switch closed I measured flat gain with no phase shift at all.

My advice to you is build it as two separate channels, tie up the unused op amps as spares, don't leave the inputs floating. So each channel will use three amp packages but only use three of the six amps. Use a dual 50K pot for balance to separate each channel. If you really,really want to do away with the balance pot just put a 25K ohm resistor to ground where the pot would go to simulate the pot.

Oh, and use 1% metal film resistors such as the YAGEO's from DIgikey. VERY low noise,stable, and measure better than the 1% spec'd. 1/4 watt size is fine and fits right onto standard board spacing such as Rod's boards. I don't advise wiring point to point on a piece of perf board as you might have oscillations from wiring too close. But it might work OK, you never know.



Thanks again.

Good luck

Last edited by jrockhead; 30th March 2011 at 06:15 PM. Reason: CRS
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