My Take on an SRPP Output Stage

[amc184]

I just finished a major (by my standards) project today, an SRPP (totem pole) triode output stage for a CD player. This idea has been around for a long time, and this has been done before. I was particularly inspired by Lukasz Fikus and his site Lampizator.

I took the core SRPP circuit values from Lukasz's site, and combined it with a straightforward LM1086T-ADJ regulated filament supply and a small signal relay based mute (to replace the functionality of the muting transistors in a player's original output stage). By default it suits a TDA1541A, but it could be adapted to suit almost any DAC. This DAC is still my favourite, not just for it's sound, but for the beautifully made radial arm laser mechanisms it tends to be paired with.

I installed it in my otherwise stock Rotel RCD-855. The sound is pretty good (don't ask me to describe it more than that, airy, shimmery, what?), but not light years from what can be achieved with a carefully designed opamp based stage. It's dead silent when it should be, with a background noise level lower than the integrated amp I tested it with, which I'd credit to the tight layout and good filament supply. I'm really pleased, as this is the first project I have done with tubes, and I really had no idea of what to expect.

The only disappointment would have to be the muting relay, it's not as effective as I'd like it to be. There is still a slight power on crack, and a medium sized power off thump. I don't know if the relay is just a bit slow, or that the mute signal that drives it (which was designed for an opamp stage) just isn't present for long enough to cover the undesirable noises in a tube stage.

The total cost for this project wasn't actually too bad, probably around NZ$200. The PCB and R-core transformer were the biggest costs.

I designed the PCB using DipTrace, a program I would recommend. Some basic specs for the PCB:
- 140 x 50 mm in size
- 101 holes
- 2 layers

As for component choices, my selections were:
- Resistors : Japanese made carbon compositions (unknown brand), mainly for aesthetic reasons (a poor justification), but also because I think they are part of the 'tube sound'.
- Electrolytic capacitors : Nichicon PS and PT, good quality capacitors at a reasonable price. I did have a couple of 'teething problems' (actually small fires) with the PTs in the HV supply, as they weren't very fresh and I ran them pretty close to there ratings (320 V on 400 V rated caps). Forming them by running the whole stage for a while on my 240 V to 120 V stepdown autotransformer solved that problem though.
- Film capacitors : Illinois Capacitor's MKP series, good and cheap!
- Semiconductors : On Semi diodes and a National Semi regulator, I like to stick to brands I know and trust.

Anyway, here's some pictures and a brief write up / notes document I did on it (which make be a little bit out of date in places):

[Glowbug]

Very cool

[amc184]

Thanks Glowbug.

I've had it up and running for a couple of eight hour stretches now, so I'm fairly satisfied that it's going to be reliable. I think the major change I'll make for the next version is adding a jumper so that it can use ECC83 / 12AX7 tubes, instead of just the 6N2P it does at the moment.

[NICKEL]

Great work!
If you are making any more boards, I be interested in a couple of sets.

[amc184]

Thanks Nickel.

I haven't really got any spares, since it was a prototype I only ordered a few. However, if there is enough interest I may consider doing a group buy. Boards like this aren't that expensive, around US$4 per single channel PCB if there are a few keen people, or less if there are more than a few.

Whereabouts in New Zealand are you based?

[NICKEL]

Hi amc184

Based in Wellington.

I'd be in for a couple of sets. Lets hope there's a bit of interest.

Cheers

Nick

[poynton]

What is the transformer ??

Andy


.

[amc184]

A nice little R-Core from this website: VT4C. Dual 115VAC primaries, dual 220VAC (50mA each) and 6.3VAC (800mA each) secondaries, total power handling 50VA.

They're US$22 each, and shipping to New Zealand was US$9, which I thought was pretty sharp. They're compact as well, which makes fitting the tube output easier.

[gabdx]

I want to build a tube SRPP output stage and I need some good regulator,
I seek help in choosing the best regulator , with less ripple and high noise rejection and stability etc ;

1. Janus regulator
2. Aikido solid stage regulator , the da-1 or something
3.diy-gene on ebay : LS-57, or LS-65
4. Lampizator style with 5U4C diode , CL(C//R)LC --- or CL(C//R)RC
5. Lampizator style with semiconductors diodes like vishay + caps+res

[adamus]

how about the salas hv shunt as well