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Old 30th November 2003, 02:16 PM   #1
Rarkov is offline Rarkov  England
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Default Tube Preamp for Apox Volume Control

Is there are nice low voltage tube preamp that would be suitable for this? Just something simple like a 6922 or equivilent.

What I'm after, is a great sounding preamp, for not much money (after paying for metal & apox boards). Can I just copy the buffer part of the JLTi gainclone? What I was thinking was two 6922 tubes with each side paralleled per channel for a total of two tubes.

Does this "sound" OK?

Thanks
Gaz
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Old 30th November 2003, 02:54 PM   #2
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Default Re: Tube Preamp for Apox Volume Control

Quote:
Originally posted by Rarkov
Is there are nice low voltage tube preamp that would be suitable for this? Just something simple like a 6922 or equivilent.
Low voltage ccts sound pretty ordinary ime, especially with the 6DJ8 family. If you need to go the LV rout for budget reasons, already having the power trans' etc, build one of the Pass designs

Quote:
What I'm after, is a great sounding preamp, for not much money (after paying for metal & apox boards). Can I just copy the buffer part of the JLTi gainclone?
Sure. This would be a better route, but it will have a gain of about 0.9.

Quote:
What I was thinking was two 6922 tubes with each side paralleled per channel for a total of two tubes.
Don't parallel tubes unless you have an insurmountable technical reason for needing to do so.

Quote:
Does this "sound" OK?
It might.

Some questions for you.
- Do you want or need gain?
- Do you really need a buffer stage?
- What's your budget?
- What bits do you have in your parts box that you can use towards this?
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Old 30th November 2003, 04:19 PM   #3
Rarkov is offline Rarkov  England
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Thanks for the reply...

OK, to answer you questions:

I would like some gain but obviously not too much. I like to have the option there if I need it.

I do not know whether I really need to buffer it, but people are complaining of horrible output impedances etc etc, and no-one seems to use these APOX attenuators on their own. Anyone know a reason for this?!

My budget for the preamp stage is maybe £60-£100 (GBP).

Finally, my parts box will hand out caps and two 6922 ceramic bases! Not much I'm afraid. I tend to order when needed.


Thanks Brett...
Gaz
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Old 30th November 2003, 04:49 PM   #4
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Quote:
Originally posted by Rarkov

I do not know whether I really need to buffer it, but people are complaining of horrible output impedances etc etc, and no-one seems to use these APOX attenuators on their own. Anyone know a reason for this?!
My experience with Apox is very limited and I was playing with complete kit for one day (or two) only, but what I found wasn't suitable for use directly at the chip amp input (if this is what you after).

The main reason is that impedance is all over the place, it jumps from 50k to 6k to 12k and it seems like achieving proper steps was the main concern.

I'm currently using NI GC and the input impedance never goes above 22k. This means that if you are using any pot (with value greater than 22k) it shunting part should always be in parallel with input imp setting resistor, otherwise the offset increases substantially. Of course all this has effect when no coupling cap is used (and I prefer to keep it this way). This requires switching attenuator to behave in a more linear fashion for proper steps.


I believe that the best use for Apox (as it is now) is at the output of the preamp and using low value resistors for impedance switching. In this way, the variations won't be much influential on the system performance.
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Old 30th November 2003, 09:19 PM   #5
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Hi Peter,
Your posts were indeed some of the posts I was referring to. I do like the look of the Apox system, so I'd like to think that I can iron out some of the problems...

Firstly, I am building some Class-T Tripath stereo amps (mainly to be used in bridged mode - but give the flexability). Until I do these, this preamp will replace a Velleman Preamp (K8021 rings a bell). My amps at present are my trusty OptiMOS 180W monoblocks (as I haven't finished the second GC3 yet...Can't really be bothered at the moment either)...

Anyway - that is the system as it stands (oh the front end is a CD-PRO2 with TDA1543 DAC)

Should I buffer the output of the Apox to cope with the impedance changes? Maybe a FET if not a valve? I had considered a P1.7 but two things stopped me. One is the price and the other is its (seemingly) complex circuit, but I haven't been able to find a schematic...

Thanks,
Gaz
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Old 30th November 2003, 10:04 PM   #6
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Hi,

Quote:
This requires switching attenuator to behave in a more linear fashion for proper steps.
Yes, and the only way to assure this happens is to use a pot with constant impedance at all positions. (series types)

Shunting pot arrangements do not achieve this.

Quote:
I'm currently using NI GC and the input impedance never goes above 22k.
Unless the preamp can drive a virtual short, I'd be more concerned with what happens below 22K not beyond it.

Cheers,
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Old 30th November 2003, 10:11 PM   #7
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The thing is if input resistance increases above 22k, DC offset increases as well. And this is quite important.

If you are talking about Pass P1.7, I don't think it's a complex circuit. I will build one soon. Here's the circuit:
http://www.passlabs.com/pdf/aleph/apserv17.pdf
Apox should work well with this preamp.
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Old 30th November 2003, 10:15 PM   #8
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Hi,

Quote:
The thing is if input resistance increases above 22k, DC offset increases as well.
I see...I was just spouting general tube wisdom.

Where's that soapbox again...
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Old 30th November 2003, 10:54 PM   #9
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Peter,
The Pass P1.7 is the circuit I was talking about. There is a thread somewhere of that exact preamp (Apox & P1.7). I must say, it looks wonderful! However, I do not believe I need such a complicated preamp, after the apox attenuator, do I?

After all, a preamp should simply act as a buffer (creating a constant output impedance regardless of the input impedance) and provide a small amount of gain, at the low THD possible.

Maybe even a high quality opamp, or discrete opamp may be the best way to go?

Thanks,
Gaz
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Old 30th November 2003, 11:03 PM   #10
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Apox DOES offer a constant impedence board.
It just takes a lot more resisitors.

Check it out on their website:

http://www.apoxcontrols.com/apoxVolumeBoards.htm?

It's the APOX 2
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