vacuum tube active stepup for moving coils revisited

[Doc B]

We have been doing some research lately that revolves around determination of what mechanisms cause what kinds of noise in low level tube amps. And we seem to have had a bit of a breakthrough with a very nice sounding moving coil step up circuit taking shape. I'm not ready to share a lot of detail yet beyond saying that, so far, the biggest issue regarding noise seems to be tube choice. Some types are much more quiet than others, and we seem to have found one that is running very nicely.

 

[Clark B.]

Sweet!

This bodes well for my investment in ordering a Woody from Pete Riggle.  He likes the DL-103 alot on them (and my own listening confirms this).  But I was wondering what sort of newfangled contraption I was going to have to pick up off Epay to step up the damn thing.  I've only used MMs so far in my own systems, but have generally preferred MC's.  Just never taken the plunge till recently.

But now that y'all are working on this, I'll hang tight and save up for the step up trans.

I might have to call up friends Dave and Jeffrey to see if they'd put together another one of these for me.  Got to hear it after hours at RMAF 2012, I think we stayed up till 5 AM that night till security came in and kicked us out.  Come to think of it, I think their diy cartridge as based on a 103...

Cheers,

Clark

 

[denti alligator]

If this becomes realized, I truly hope you make it available as a step up for the existing Sed/Reduction and Eros and not as a totally new amp. Would be great.

 

[Doc B]

The concept is a step up device for any phono preamp, not just Seduction/Reduction and Eros. This is still being developed, so nothing is particularly firm. However the initial finding is that gain is quite high, perhaps 15 dB more than my 1:10 step up transformer, while noise is very low. We're getting a whole lot of signal from my .35mV Haniwa cart. So it seems it could work best with low output cartridges.

 

[denti alligator]

Holy tubes, Batman, that would be sweet.

 

[Paul Birkeland]

I'm hoping to have a mobile prototype version with minimal features to take home and listen to over the weekend

 

[pboser]

Clark's choice of the Woody tonearm seems a match made in heaven for Bottlehead equipment, given the history of the names of Bottlehead stuff, starting with S.E.X. and going (descending?) from there.

 

[johnsonad]

It would be really nice if it could be used with tape heads too.  Please keep us in mind!  I would love to get another 10 to 20 dB before my Eros tape.

 

[earwaxxer]

wow, I'm impressed with the DIY cartridge! That flat blows my mind (that together with the 'woody' of course). As Greg would say "Its alive!".

 

[sl-15]

i was about to ask, with all the numerous great developments lately, if the tube step up idea ceased or is still in the running. very glad to hear you guys are working on it and making progress. looking forward to something like it. thanks!

 

[xcortes]

Cool!

 

[owhite67]

Hey Doc, have you researched nuvistor tubes?

 

[Grainger49]

Hey Doc, have you researched nuvistor tubes?

Hmm.... I had some thoughts about that a few years back.  I have a pair of them but no sockets.  They are reported to be high gain and low noise, maybe because of the metal shield. 

I would like to hear Dan's thoughts too.

 

[Doc B]

Nuvistors are nice for the right application. We use playback electronics with nuvistors (Ampex MR70 electronics) for duplicating in the Tape Project. However for this project the 6C45pi has floated to the top. An application like this is a great one for those obsessed with getting all the noise one can out of the circuit. The 6C45pi goes a long way in helping keep the noise down, as its noise contribution is very low.

Today we made some very good headway. The input signal level involved is low enough that regulator noise can become an issue as it gets amplified though the gain chain. We spent the day working with various regulator circuits, finding what we felt was the best combination of low noise and good sound. We also worked up an idea for variable loading and variable gain that should make the step up preamp quite versatile with a wide range of MC cartridges and phono preamps.

By day's end it was sounding very good. I think we should be able to move from the POC prototype to the functional prototype in the next couple weeks.

 

[johnsonad]

It would be really nice if it could be used with tape heads too.  Please keep us in mind!  I would love to get another 10 to 20 dB before my Eros tape.

Dan, or team, can you answer this please?

 

[Paul Birkeland]

It would be really nice if it could be used with tape heads too.  Please keep us in mind!  I would love to get another 10 to 20 dB before my Eros tape.

Dan, or team, can you answer this please?

IIRC, most tape heads are going to put out a few mV, and the Tape Eros is optimized for this signal level.  You have to remember that the Eros circuit can be overdriven, with the Tape Eros probably not wanting to see more than 2-3mV.  If your tape head is putting out 1mV, you would need to burn off nearly all of the signal out of the active step-up not to overdrive the Eros and end up with a lot of extra distortion.  This would maybe net you 3-6dB, and certainly nowhere near 10-20dB. 

Extra gain from the Eros would have to be obtained by a 3rd stage, a step-up transformer, or a fresh set of tape heads.

-PB

 

[johnsonad]

Hmm... Thanks PB.

 

[Paul Joppa]

To comment further, tape heads vary in voltage sensitivity and optimal impedance. All our tape head preamps are designed around a moderately high output tape head (250mH IIRC) with something in the vecinity of 100kohms load impedance. Really high impedance heads (500-1000mH) have some treble limitations, and low impedance heads are optimized for solid-state preamps. If you have a low-impedance head which you are committed to, then something like what we're looking at can make it work with all-tube electronics, but it adds a stage, so (in my humble opinion) it's better to use a high-impedance head with tubes if you can.

 

[Doc B]

FWIW I was doing demos all day today and had digital, phono and tape all running. My Nagra has a fairly high output head. The output from the single ended RCAs of the Repro, which would be the same output level as an Eros, was WAY louder than the phono with the new active step up, or the DAC. Like 10 dB or more. So I'm inclined to suggest that you might want to look for a higher output playback head.

 

[johnsonad]

My 1/4" 2 track Flux Magnetic head has plenty of output. It's the 1/4" 4 track stock Otari head I would like a few more dB of gain out of for old comercial tapes. I can live without it but it would be nice. John French wasn't able to find a higher gain head that would work with the Otari.  It's been a while since I asked him and maybe I'll try again. Maybe a Nagra head will work....