Driver amp

[Max N]

I thought I should post a few notes on the driver amp I took to Owston. I'll give a bit of background first. A few years ago I decided I wanted to build a 211 push-pull amp. Neal also fancied the idea so we agreed to collaborate. I wanted to experiment with various different driver topologies, so I decided I would split the amp into a 'driver amp' and two output monoblocs.
I feel I am at risk of stating the bleeding obvious, but just to clarify what I mean by a driver amp:

Normally, amps (if not integrated) are split at line level. So source switching, volume etc and possibly buffering are in the pre-amp, and there is a line-level interconnect to a power amp or a pair or monoblocs or whatever. The power amp probably includes a gain stage, a driver stage, and possibly a phase splitter, as well as the output stage.

With a driver amp, the source switching, volume control, gain stage, driver stage, phase-splitter etc are all in one chassis. The power amp (or monoblocs) then consists of just the output valve(s) and transformer. The obvious drawback is that the interconnects are now at high voltage. You have to decide if you can do this in a legal, safe way, not invalidate your house insurance etc. One solution is to make sure that no voltages are present if the interconnects are disconnected. Anyway, just to be clear I am not recommending this and if anyone decides to do it they need to make sure for themselves it is safe and legal etc.

For me the main motivation is the flexibility that this gives. It means I can experiment with different driver topologies more easily. You may be thinking, why not just breadboard everything until you have settled on a topology, then build a 'normal' amp. My problem is that I don't have my own listening room. So if I want to do any kind of extended listening then the amps need to be in the front room, and I'm not going to put breadboards into the front room. I can also experiment with different output valves, and have a ready made driver stage for them.
Ultimately I may have a variety of different driver amps and output monoblocs.

There is at least one other advantage:
211 power amps tend to be a bit big and heavy. A driver amp and two output monoblocs are a more even split in terms of weight and size.

Driver topologies I plan to try:

300B PP driving lundahl LL1671 1+1:1+1 or 2+2:1+1 interstage

6sn7 differential amp driving 6sn7 cathode followers DC coupled - like Morgan Jones' Crystal Palace amp

5687 driving PP 1+1:1+1 interstage - this is the one that was at Owston

A few specifics about the driver amp as it was at Owston:
Going back to the original 211PP plan, Neal and I both like the original WAD 300B PP which of course has SE:PP interstage transformers. Neal contacted Andy Grove who worked on that amp, and asked him if he would design some transformers for our 211 project. Neal, you had the actual conversations with Andy so please correct anything I have wrong here. As I understand it, Andy wasn't keen on the SE:PP IT idea (I think he took over the 300B project for WAD after the topology was already fixed. In fact Tim de Paravicini may have done the prototype - but I digress). To get back to the point, Andy suggested going with a PP:PP interstage driven by a 5687. We agreed, so Andy designed the interstage and output transformers and Neal arranged for Morite to wind them. A big thanks to Neal for doing all the organising with Andy and Morite.

Progress on the 211 amps has been slow for both Neal and I - in my case its pressure of work, and I suspect similar for Neal.
I was determined to have something to demo at Owston for once, so I thought I would try to lash up the driver amp. Insomnia was my friend, so I was able to just about get it finished by working nights out in the shed. The enclosure at Owston was something I built originally to house the K&K combined DAC and linestage kits. You can see photos of it as it was back then here:

http://www.geocities.com/wulfie22003/Preamp/Preamp.html

I ended up selling all the K&K internals, so the enclosure was sitting empty. When Steve posted the 'safety notice' in advance of Owston, I decided I shouldn't bring a breadboard so I built the prototype driver amp into the old enclosure.

I obviously needed an output stage so I took my WAD 300B PP clone and disabled the gain and driver stages. I then wired from the hot and cold of the input XLRs direct to the grids of the 300Bs, and the ground pin of the XLRs to the bottom of the 300B cathode resistors (earth)

So, just to re-cap on the topology:

The driver amp is just a pair of 5687 triodes in push-pull, loaded with the primary of the PP interstage. The secondary of the IT including the centre tap goes via balanced XLR into the output stage amp.
As it stands, the driver amp has no gain stage and no phase splitter. It just about worked Ok driven from the balanced output of a CD player and driving Steve S' speakers. It struggled for gain when driving Nick's speakers, and sounded horrible for some reason.

The plan now is to check everything and see that the driver amp and power stage are working OK (I didn't have time to check anything beyond a few voltages before Owston). Possibly reduce the dissipation of the output amp and buy some new 300Bs if mine are shagged. Finish the driver amp (integrated DAC, gain stage, phase splitter).

A few specifics about the interstage transformers cut and pasted from the 'Interstage' thread:

They are PP 1+1:1+1 and were designed to be driven by a 5687, but similar valves should obviously work. They are bi-filar wound, so bandwidth should be good (I'm hoping to measure them asap and will post the results). The bi-filar winding means that there is a limit to the voltage between primary and secondary, so you need to keep the HT on the driver valve sensible, and you probably don't want to negatively bias the secondary (ie you may be limited to cathode bias on the output valves). Depends how far you are happy to push things, but you're obviously relying on the enamel to isolate primary from secondary.
The main downside was the interminably long time it took Morite to wind them, but that's understandable since it was such a small order.
I don't want to drop Neal in it, but if there was a lot of interest he might be persuaded to organise another batch


It may be some time before I can post again, I've been up half the night typing this (cue violins), but I thought I should post something.

Finally, thanks once again for everyone who said nice things about the amp - I can't tell you how nervous I was about playing something of mine for the first time.

[Neal]

Thanks for the write up Max and the details on the Owston setup, sorry I missed it. The ITX where designed as you described, AG's idea was to use a Concertina splitter ahead of the 5687 the ITX out to 211.

I'll try and post the original cct idea later.

[Neal]

Here's an early cct idea, ignore any values and the choice of input valves!

[Nick]

Thanks for the info. I was about to comment on the ckt, but it was about the input valves, so I wont

[Neal]

Please do Nick! I would like to get feedback on the best route to take...

[Nick]

Well my only thought was that by chhosing to use the first valve direct coupled to enable the concertena, it was putting it into what looked like a poor area of the curves. One of the things I have agaist direct coupling.

[Neal]

The 6SN7? Yes I agree. Andy suggested an ECC83 as the first valve, one half as the input feeding the second configured as a Concertina. He felt this would give a more pleasing balance of harmonics...