[pboser]

I just picked up a Versa-kit headphone amp from an estate auction.  I've tested it out and it seems to work fine.  The Noble volume pot seems unusual - it only seems to have about 90-120 degrees of rotation. I don't know if that's normal, or if something strange is going on.

I'm thinking that I'd like the option of using it as a line stage, and so I looked through the Audio Basics archive.  I have a couple of questions:
1)Is the Versa-Kit the same as the Omega II Line Preamp Kit and Headphone Amp in the August and September 1992 Audio Basics?
2) I see some differences in the schematics -can I add an additional pair of RCA jacks and use the headphone amp as a line stage without modification?

Thanks in advance,
Pete

[avahifi]

The headphone amp and line preamp shown in Audio Basics are nearly identical.  You can download the build plans and schematics for both at our web site: avahifi.com.  The headphone amp is in issue 8/92.pdf, the line preamp 9/92.pdf

The difference is the gain set.  The preamp has a voltage gain of 10 done with a 10K resistor in series and a 1K resistor to ground in the feedback loop.  The headphone preamp has a gain of about 3 using a 4.7K resistor in series and a 1.5K resistor to ground.

Either can use the AD843 opamp, but a Burr Brown OPA627 would be a better choice for either.

Note that our custom circuit board for either application is long out of production and no longer available.

Regarding the volume control rotation issue in the one you got from an estate auction, the limited rotation is a defect.  Possibly the shaft is simply sticking in its bushing.  It should rotate over 270 degrees.

Frank

Frank

[adydula]

I see the Signal Transformer is still available.

Any idea if the pot is still around?

What value?

Might just wire one up to make the winter months more enjoyable!

Thanks,

Alex

[pboser]

Hi Frank,
Thank you for the reply, and for the gain info. I will give it a try as a line stage and see if I would prefer more gain. ( I will also see if the volume control needs to be replaced too.) And I will replace the opamps too.
One more question- from the schematics I see that the output of the line stage is a 560R in series and a 24k to ground, and the headphone amp is just 100R in series. Is there any need to change it for line stage use?
Thank you again- it’s very generous of you to spend time on this for me.
Best regards,
Pete

[avahifi]

We have a replacement volume control available for $30.00 plus $10.00 shipping.

The 560 ohm series output resistor or 100 ohm value is OK for the line preamp,  but must be the 100 ohm for headphone use.

Frank