Balanced Zen Line Stage Pre-amp R15 Sound Quality

In the middle of testing 1 channel of my "Balance Line Stage" pre-amp and would like to know if others have experienced the same problem when R15 is changed.

The pre-amp is running fully balanced and my cdp is outputing +4 dBU. Also the voltage drain rails on the mosfets (IRF610) are near 40 volts. I've omitted pots. P3 & P4 and using a quad ganged 10 Kohm switched attenuator for P1 & P2. Now for the beef.

I find that the sound quality is MUCH better when R15 is say approx. 200 ohms or LESS (less than 25db). However, this is contrary to what is show in the "performance notes" (links below). Normally there's more distortion when the gain increases (as the R15 value increases)?

When R15 is set say 400 ohms or more, the pre-amp seems to clip / distort the signal more readily when the attenuator is turned up. But at R15 at 124 ohms, the attenuator can be set at a higher volume level before clipping/distorting. I figure i'm not getting enough voltage swing from the circuit when the gain is low?

Following links for reference:

http://www.passlabs.com/projects/bzpre8.htm

http://www.passlabs.com/images/projects/sozpf9.gif
http://www.passlabs.com/images/projects/sozpf10.gif

What do you guys have for R15 set at? Any recommendations that I can do? I'm quite happy leaving R15 at 124 ohms but if there's a way to gain better sound and performance.. please let me know.
 
I'm in the process of getting all the parts together to built this pre-amp. I'll let you know after I built it. But for now, could you tell me where you bought the 4-gang
potentiometer, how much and the brand name? I'm going to use two 2-gang ALPS potentiometers but I would prefer 4-gang if it's very reasonably priced.
Your post did mentioned that it's a switched attenuator, so I assume it's one of those $300 DACT pot?
 
4 gang - attenuator

fcel, the attenuator i bought is made by Electroswitch. Their website is at http://www.electroswitch.com - specifically
http://www.electro-nc.com/c7.pdf

I've looked at the DACTs before and IMHO, they are quite simply a ripoff if they're ONLY used in an active preamp. But to be used as a passive preamp ; then that's a horse of a different colour - cost MAY be justified.

The ones I bought were on sale - I think I paid $40 usd each + taxes/shipping. One of the reason for buying a switched attenuator is because i'm using the pre-amp fully balanced. But if in the case of being single ended, I think almost any cheap pot will do since the +/- source/output leads do not have to be in precision gang settings.

Since I built a voltage divider, ideally I wouldn't mind getting an 8 ganged attenuator (if they make one?)- that way instead of the signal being passed through several resistors in the attenuator, I could just have the signal pass through only 2 resistors for each lead (4 per channel). Of course I could use 2 separate 4 gang attenuators (1 for each channel) but I don't like the idea of using separate left/right channel dials.

Now to see if anyone has played around with the values of R15 and had the same findings as I did?
 

grataku

Member
2000-12-31 9:31 am
-
Boy, everybody and their grandpa are building this pre-amp! That's good, we are all in this together!
I am still waiting for the transformer, so I won't be testing for awhile.
Did you see anything strange on the scope? Is anything clipping (inputPRE -outputCD, or after the power amp)?
Unless you are GRollins, hearing the difference between 0.01 and 0.1 THD is going to be tricky. ;)
So there is a chance that what you are hearing is something that is being driven too hard or to clipping.
 
I am struggling to find decent competitively priced stepped attenuators/dual gang pots/2 pole >12 throw switches suitable for volume control for either of the pass line stages. I live in Australia. Does anyone know of any local or international (who will freight anywhere) companies who might fit the bill.

Thanks for any help

Dan
 

Nelson Pass

The one and only
Paid Member
2001-03-29 12:38 am
It's interesting that R15 does in fact vary the sound
quality. With Aleph P's, there was a consensus among
users that the 1 Kohm pot used in that spot sounded
best at about the 3 o'clock position, which corresponded
to about 200 ohms. With larger resistance values, the
measured distortion and noise was better, but people
didn't prefer that in this particular circuit.

I have a hypothesis that devices have an optimal
working range sonically, and that often you need to
"exercise" them in a larger signal range than is
otherwise necessary in order to get the best sound.