New 300B pre-amp?

[Paul Joppa]

The input resistance of the level controls should be high enough for the sources you will use with it. For use with Eros or Seduction, that means 30-50K minimum so that's our criterion. Other sources will have their own requirements - a few tubed components would prefer to see 100K or more, while most solid state sources are happy with anything above 10K.

The output impedance of the level control should be low enough for the input capacitance of the tube; in this case 50K maximum. For a normal setup with a single potentiometer, its output impedance is never more than 1/4 of the pot's total resistance, so a 200K pot would be the maximum.

[Paul Birkeland]

Team BH, what is the attenuator resistance range for this preamp? Hopefully 10k or 50k will work? I really would like a remote, (for once) and have an ideal in mind.

The stock configuration, at least as it is drawn now, is about 57K.  The upgraded attenuators, as planned, would have a variable input impedance that would vary, but be no less than 50K at any position.

For aftermarket attenuators, you can kinda use what you like, but I would stay closer to 50K if possible.  Also, there is a pair of ~12V DC rails in this preamp with the negative end grounded and some surplus current, so you likely won't need a wall wart to add a remote attenuator (though you might if it has a motorized pot). 

[johnsonad]

Thank you PJ and PB, that's what I was looking for. I'm considering building mine up with the Bent Audio 50k shut modules, input selection, display and remote units. It's a little pricy but very convienent   I had one of John's TapX preamps for a while and really enjoyed the ease of use though the sound didn't work well with my system at the time.

[Gerry E.]

Hi:

Will the contruction manual or purchase options include a one input only option (no source selection)?  Due to less switches, wiring, etc., can a single input be beneficial?  Thanks.

Gerry

[Doc B.]

Hi:

Will the contruction manual or purchase options include a one input only option (no source selection)?  Due to less switches, wiring, etc., can a single input be beneficial?  Thanks.

Gerry

No. Just leave that stuff out if you want to, but it isn't going to change the sound much.

[johnsonad]

Hmmm, I got to thinking again which is dangerous.  What are you thoughts of using a set of Mikey's 10k/10k transformers on the input side to further isolate the ground and use my fancy 10k P&G pot?  Would this potentially offer any futher input noise reduction?  I have a three way Gold Point switch for use also.....

Thanks!

[Paul Joppa]

In our experience, Mike's transformers have been the best sounding (I think it was a 15395 clone). But there is always some bandwidth limitation from transformers, so no transformer always sounds a hair better. Transformers do provide ground isolation and hence lower noise (hum and buzz); they allow balanced lines, also good for hum and buzz. The downside is hum pickup from magnetic fields; usually you want at least one and up to three concentric mumetal shields on an input transformer. (Normally they will have copper Faraday shields between each mumetal layer ...). This starts to get expensive and difficult!

[johnsonad]

Thanks PJ, worst case I put some double sided tape on them and move them around for the least hum pickup from the power transformer.  One in theory could just get some of that Mu metal tape/shield and wrap the transformers well too correct? 

So the 10k/10k B7's would work well then?  Anything else I would need to do?  I see it as input>switch>B7's>pot correct?

[Paul Joppa]

The tape is not as effective as a fully-enclosed can, and you must be careful to not manipulate it more than necessary. I think you can get around 15dB from careful taping, vs 30dB per can shield. Hopefully a combination of careful placement and tape will make things good.

You circuit is correct. The pot provides the correct loading for the transformer so that it performs optimally.

[Doc B.]

I did some experimentation with shielding small signal transformers a while back. It is very tricky. I ended up using a combo of moly perm tape, mu metal can, copper tape, aluminum box. The final step was to try putting it all in a steel can to fend off the stronger magnetic fields. And it still picked up hum.

[johnsonad]

Wow.... So you are saying it's worth a try but expect hum.....  Are there other 10k/10k input transformers out there that would work well in this application and is it even worth persuing for a non-balanced configuration? 

[Doc B.]

Altec 15335s seem to work well. I'm sure they aren't as nice sounding as Mike's, but they do come with well worked out shielding. You would have to come up with a way to mount them, since they have octal bases.

[johnsonad]

Thanks Dan.  They are 15k transformers and the P&G pot is 10k, not an ideal match if I understand things correctly?

[Paul Joppa]

No, not ideal - but pretty darn close. I wouldn't sweat it - the lower resistance will reduce the Q of the HF rolloff resonance and the greater damping may well actually sound better. You can always put 5K in series with the pot; you'll only lose 3-4dB. Try it and see if you hear any actual difference.

[johnsonad]

Thanks Paul.  I've got both sets of transformers and will give them a shot.  Please post if you guys find a set and position that ideally match the pre-amp.