Tube Phono Amps

[Primalsea]

I've been thinking about buliding a tube phono amp for a while but only started to look into it seriously a few days ago. Most of my projects have been solid state, the only tube project is my hybrid preamp that has a 6dj8 Buffer in it. A tube phono amp seems like a good 2nd project.

I've looked around on the web but have been unconvinced by many of the projects I've found so decided to design my own. What I have at the moment as a soild state design that I hope to carry over to a tube design.

what I have so far is a discrete opamp that actively equalises the first part of the RIAA curve (20 - 1Khz). However the feedback loop is first attenuated before the equalisation can take place. You will see on the posted circuit the resistive attenuator in the feedback loop which is then follwed by a Complimentry Feedback Pair (CFP) follower. The CFP follower isolates the resistive attenuator from the RIAA equalisation network that follows it.
This gives a first stage that actively handles the first part of the RIAA equalisation as well as having all of the gain of the phono amp. (the current resistor values set the gain is too low at the moment, it needs to be changed).

The 2nd part of the RIAA equalisation is passive and follwed by a buffer only. The idea here is that this will lower the noise floor for frequencies over this part of the RIAA curve.

I have read that phono stages are a popular topic here with some people and that there has been some shoot outs in the past. I'm hoping that someone will point me in the right direction on turning this SS design into a tube design.

A good start would be for someone to let me know what tubes would have enough gain for this??

[Andrew]

Hi Paul,

Well I need to look at your design, but first off let me say I'm not a fan of active eq. I'd go for passive eq, either in one stage or two.

Both the "one" and "two" stage approaches have merit and their followers. I've never done a two of my own but have built the Hagerman Cornet which is a two stage affair. Jim Hagerman posts his circuits on his web site and sells a PCB, this is a nice two stage design, similar to the old WAD Phono II, but I had hum problems with it in my system when I used step ups.

The dogs wotsits seems to be LCR eq, but this hard to do since the LCR eq is a bugger to drive, very low impedance, nominally 600R. Most valves don't like such low impedances and if its not matched, then the EQ will be wrong. Now Thorsten L, for whom I have much respect, thought I don't always agree with, designed a "one" stage CR-based design using an ECC83 followed by an ECC88 which, for the money, is really really hard to beat and can be made even better with a bit of judicious tweaking.

Getting a phono right is very hard since the eq is difficult to get spot on. I use Spice to help get the eq right, but after that you need to test it as well, on a speccy analyzer or some such. I have to say even some of those published well-kown circuits on the web sometime have poor eq, and in some others the eq can vary as the valve ages and its mu chages hence Zout changes.

Its not simple...but don't let me put you off, I'm happy to help.

-- Andrew

[Phil Y]

For what it's worth i have just built a new phono stage based on the phono II and i think it sounds very good. Certainly miles ahead of the SS one i have been using. I have been listening to more music since i finished it which for my money is the best test there is. The audio part is slightly altered Phono II but the power supply is totally different as i used the chassis (and PSU) from an old line level pre amp. Yes i'm just plain lazy. It does not look too pretty but it does the job and the PCBs are still available.

[Andrew]

Hi Phil,

Looks good, nice the WAD Phono II and pleny of tweaks to be had from the folks here on the BB as well.

cheers,

-- Andrew

[Andrew]

Hi Paul,

I just re-read my last para, I wrote this in a hurry whilst keeping my eldest occupied with jigsaws during the youngests dinner (8 months and 2 years respectively). On reflection it seems a little presumptious, for that I apologize, for all I know you might have both the skills, expertise and knowledge to make a very good job of your proposed design.

What was I trying to say? Well simply this, phono's are a really good challenge to take on since they is a big learning curve, after three and half years of doing not much else, valve-wise, I'm just begining to realize what I don't know, rather than what I think I knew, if you see what I mean.

cheers,

-- Andrew

[Primalsea]

Phil,

Thanks for posting the pics. I'd be interested to have a look at the circuit of the WAD phono amp. Is there one posted anywhere??

Andrew,

I'm a bit confused as I'm not sure if you have looked at the circuit I posted. I know what you mean about keeping the little ones occupied. They only seem to want attention when you start to do something you need to put your mind to.
A bit like the adult equivalent of not saying anything until you start to read your hifi mag AND THEN start having a conversation with you (but I'm over THAT now).

I've played some more on my simulator and I think I might struggle to make some form of tube opamp with enough gain without having to have additonal gain stages. I need 40db for the equalisation and another shed load for the output to be something reasonable like 400mv.