RIAA preamp with DHTs

[mogliaa]

Hi everyone,
Not sure if I'm about to ask a stupid question, but here I go. Does anyone built a RIAA preamp with DHT in all stages? I'd thought that the gain is an issue as will require more stages to achieve the right gain. Is there anything in particular the would rule out DHTs for this?
Thanks
Ale

[SY]

It can be done, but not trivial if you want good performance. Gain is an issue (lots of stages or use some iron stepup), as will be noise and microphonics.

[Vinylsavor]

Hi Ale,

I have built various all DHT linestages. Mainly with tubes like UX201A and 10Y/801A.

It can be done, but as SY already wrote it is quite difficult. You need very well filtered filament supplies and need to mount the tube sockets with some damping elements to control microphonics. The tubes in the first stage will most likely need screening.

With tubes like those mentioned above, 4 stages are needed.

I plan to build an all DHT phono again in the future, using my usual chassis style. I plan to experiment with 841s which have more gain. I expect that this will be at least a 4 chassis beast, 2 signal chassis and two PSU chassis. Plus it will need a linestage with some gain.

I recommend to start with a DHT linestage to gain some experience with the handling of DHTs at small signals. Also if you have not done that yet, I'd first build a RIAA with indirectly heated tubes

Best regards

Thomas

[Vinylsavor]

Hi!

I found the schematic of an all DHT phonostage which I showed at the ETF some years ago:



Note the absence of caps in the signal path (except inside the LCR network). The filament voltages were derived from the B+ through dropping resistors.

This was quite a biest, but it worked and got a lot of praise from the audience.

My next approach to DHT RIAA will be a bit simpler.

Best regards

Thomas

[mogliaa]

Very interesting, I thought it was going to be the case :-)

May explore using IDHT as first stage then. Has anyone used the EC1010/6S53n-v high-mu nuvistor?

Cheers,
Ale

[SY]

Thomas, I well remember that beast! The saving grace is the iron: noise-free stepup.

Ale, Nuvistors have a reputation for high levels of microphonics; my somewhat limited experience with them is consistent with the reputation. So, you still have the mechanical isolation issues. The other tradeoff which needs to be considered when using high mu input tubes is the input capacitance. It can often be dealt with, but it cannot be ignored.

[mogliaa]

Hi Sy,
Thanks for that. Any recommended tubes for a first stage RIAA amp? What are the key characteristics you would look at any tube to perform this role? Low input capacitance, medium-mu, etc?

Thanks
Ale

[Sheldon]

Originally Posted by mogliaa

Does anyone built a RIAA preamp with DHT in all stages?

Yup: Unholy Alliance Phono Amp

Works great.

Sheldon

[Vinylsavor]

Hi Ale,

for a phonostage consider the whole concept first. Based on that you can make a meaningful tube choice. Selecting the tubes is the wrong way to start.

What topology are you looking at? RC, LC, transformer coupling? Anything else?

MM or MC ? For MM the input capacitance is critical, for MC not so much.

How much gain do you need?

Any preference for the number of stages? 2? 3?

Standalone phonostage or full preamp with volume control?

What kind of RIAA EQ? Active? Passive? If passive, composite or split? Or LCR RIAA? Or LR ?....

There are many prooven circuits out there. Among those my Octal Preamp. But of course many other good designs too.

Best regards

Thomas

[mwiebe]

Ale,
Steve (now Stevie) Bench has one,
6088 Based Battery Powered Full Feature Preamp
I've built it and it sounds nice. He also has one using all 5676 tubes but I cannot find the link for that.
Matt