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
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
Rarkov said:
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
Sure. This would be a better route, but it will have a gain of about 0.9.
Don't parallel tubes unless you have an insurmountable technical reason for needing to do so.
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?
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
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
Rarkov said:
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.
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
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
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.
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.
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
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
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
It just takes a lot more resisitors.
Check it out on their website:
http://www.apoxcontrols.com/apoxVolumeBoards.htm?
It's the APOX 2
Hi Variac,
I had seen this board, but not considered it, since I wanted balanced inputs / outputs and two of these boards wit the resistor pack is $280 (£162.39) !!! If I use single ended (as I always have in the past) it brings it down to a much more managable $140 (£81.19).
However, since this is only attenuation, I feel that I need some kind of gain stage, which brings us back to the original question...
What is a good small gain stage (read cheap yet audiophile ) for this system?
Thanks,
Gaz
I had seen this board, but not considered it, since I wanted balanced inputs / outputs and two of these boards wit the resistor pack is $280 (£162.39) !!! If I use single ended (as I always have in the past) it brings it down to a much more managable $140 (£81.19).
However, since this is only attenuation, I feel that I need some kind of gain stage, which brings us back to the original question...
What is a good small gain stage (read cheap yet audiophile
) for this system?Thanks,
Gaz
With P 1.7 the Apox is recommended to be used after the preamp stage (at the output) and this is mainly what it was (Apox) designed for.
You might consider maybe trying this http://www.diyaudio.com/forums/showthread.php?s=&threadid=16928
It's as simple as chip preamp comes and supposedly sounds good (although I don't really believe it)
You might consider maybe trying this http://www.diyaudio.com/forums/showthread.php?s=&threadid=16928
It's as simple as chip preamp comes and supposedly sounds good (although I don't really believe it
)
hi rarkov,
if you need a buffer with low voltage, the circuit of figure 16a in http://www.borbelyaudio.com/ae699bor.pdf should be a good choice for you, when you connect your GC in inverting mode, you will need 2 of this buffer, one before APOX 2 and one after.
DAC-->buffer-->APOX 2-->buffer-->GC
leo
if you need a buffer with low voltage, the circuit of figure 16a in http://www.borbelyaudio.com/ae699bor.pdf should be a good choice for you, when you connect your GC in inverting mode, you will need 2 of this buffer, one before APOX 2 and one after.
DAC-->buffer-->APOX 2-->buffer-->GC
leo
Hi,
I was reading through my AudioXpress mags to try and get some inspiration. It suddenly dawned on me that I had wanted to try the "Line Stage Odyssey" described in the feb and march 2003 issues. It is a transformer output line stage "with parafeed taking it to the ultimate".
It does look very compact yet appealing design. The author (as expected) raves about the quality of the line stage module.
Has anyone any thoughts on the compatability of these? It does say in the article that a volume pot / attenuator should go before the module, which is a direct conflict with what Peter has been saying. Peter?
Thanks,
Gaz
I was reading through my AudioXpress mags to try and get some inspiration. It suddenly dawned on me that I had wanted to try the "Line Stage Odyssey" described in the feb and march 2003 issues. It is a transformer output line stage "with parafeed taking it to the ultimate".
It does look very compact yet appealing design. The author (as expected) raves about the quality of the line stage module.
Has anyone any thoughts on the compatability of these? It does say in the article that a volume pot / attenuator should go before the module, which is a direct conflict with what Peter has been saying. Peter?
Thanks,
Gaz
It is not in conflict with anything. I've built BOZ and was using pot at the input, but it is recommended to use it at the output as it attenuates the noise coming from that preamp.
It is dependant on the design and pot can be placed both before and after an active module. It's just in some cases it works better one way than the other.
But I also like to use pot directly at the input of the active stage.
It is dependant on the design and pot can be placed both before and after an active module. It's just in some cases it works better one way than the other.
But I also like to use pot directly at the input of the active stage.
I'm not familiar with the article and maybe some tube guys can suggest better.
The Apox SHM doesn't provide constant input impedance, so using it at the input might be questionable. However, when high resistor values are used it might be possible. Using it at the output (providing resistance is low) whould probably be a better solution anyway. But my experience with Apox is too limited to provide satisfying enough info. Try to contact Dale and ask him. The rumor is they work on a budget version of a system, so maybe this would be something more suitable for you.
The Apox SHM doesn't provide constant input impedance, so using it at the input might be questionable. However, when high resistor values are used it might be possible. Using it at the output (providing resistance is low) whould probably be a better solution anyway. But my experience with Apox is too limited to provide satisfying enough info. Try to contact Dale and ask him. The rumor is they work on a budget version of a system, so maybe this would be something more suitable for you.
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