Sheer Audio DHT 01A preamp ... bit of fun from Taiwan

[montesquieu]

Thought I'd share some fun I've had the past few days with an interesting preamp from Taiwan.

As is discussed in another thread, for the last year or so Will from Radford Revival has been working on a custom preamp to match my Radford STA100.

Back in the summer though, it seemed a long way away .. so while I was waiting curiosity got the better of me and after spending a few weeks looking at it (and googling/finding nothing about it) I pulled the trigger on a DHT pre from Taiwan.

DHT preamps are a bit oddball, excuse the non-technical explanation but it seems to me that they use what are normally power tubes (directly heated triodes, with 4 pins) in a single gain stage.

I've had a DHT pre before (Tram2 which used either 2A3 or 45 tubes) this was an oddball thing, sold in kit form for a while by DIY hifi supply. Bit of a split personaliy - on the one hand it sounded pretty dramatic, on the other it was extremely microphonic (going 'off' ringing on occasions).

Simon Shilton of SJS fame - who had a lot of experience with this pre - fettled it for me and cured a lot of the issues, but in the end I was defeated by something pretty mundane, the integrated control unit sold as part of the kit on it stopped responding and it's pretty much impossible to disentangle it. So I swapped it on to someone who was looking for just such a project. But when on song it was spectacular. I also had another Tektron DHT preamp on test, that used 10Y's - this was also microphonic, but also had its moment.

Anyway the Shere audio was suspiciously cheap, all up bought, and shipped including import taxes and courier charge for around £400. But I had to pull the trigger.

http://www.ebay.com/itm/Sheer-Audio-...p2047675.l2557

When it first arrived it sounded fairly promising, but it had some hum and also sounded a bit glassy and bright in the upper frequencies, due to I think to some pretty serious microphony and probably some other problems as well. I gave it to Graeme Hirst (Valvebloke / Ampregen) to check over for safety/CE compliance issues, to address the microphony, and to improve the sound wherever he saw potential. It was intended to be a stop-gap before Will's arrived, but due to the inevitable, I actually got it back some time after Will's came, rendering it actually somewhat redundant.

Anyway I finally had a play with it over the last couple of nights since it got back. Graeme has done quite a remarkable job of getting rid of the glassiness, and the hum is totally gone. Microphony is much reduced (and there are plans to take this further). Graeme's view was that the power section is actually pretty sophisticated (and runs at a true 240v), but that the signal stage was let down by a few oddities in the earthing regime, in things like use of unscreened cable in silly places, and in use of oddball component choices such as carbon composition resistors. Graeme sorted all that out very efficiently and creatively.

The overall result is really quite pleasing. It has a nice immediacy and delicacy, nice tonal balance including healthy bass reproduction, and no serious microphony/stability issues at all any more. It won't trouble Will's preamp - which is truly something special - in its grasp of detail and dynamics ... Will's pre just goes up and up in volume without distortion, by which time I suspect microphony would be well kicking in with the DHT job .... but overall it's really not a bad preamp that is better I would say than most of what I've had in the house to date with the big Radford, barring Will's pre of course - that includes some interesting kit including £5k worth of MFA TVC. Not bad at all for the money and will make a useful spare/contrast. Design-wise of course it's about as far from Will's pre as you can get and still use tubes.

I have managed to source a spare set of triode tubes - the same 1920/early 1930s manufacture Sylvania 12A's as it came with from Taiwan - and several spare rectifiers. Though it runs the triodes so gently that I doubt they'll ever wear out, and the rectifier is fairly common (and cheap).

Remaining work to be done really consists of finding some large-size screening cans to assist with microphony - Graeme's been showing off his origami skils here with some improvised ones .... anyway I thought I'd share the info on it. Two inputs, one output, three tubes, one gain stage ... about as simple as it gets.

[Ali Tait]

Interesting. I remember Nick building a DHT pre, initially using 2a3, then changing to 6080. Great sound.

[Firebottle]

Sounds interesting Tom.

By the way the 6080 isn't a DHT, to prevent any confusion.

[lurcher]

Sounds interesting Tom.

By the way the 6080 isn't a DHT, to prevent any confusion.

Not directly heated no. It fixed the microphonics though :-)

[User211]

Post removed.

[Ali Tait]

Ah yes, quite right, had forgotten.

[montesquieu]

'Power' tubes - 12A, most likely late 20s or early 30s production I was told. Spares and ones in the amp are identical Sylvanias.

[awkwardbydesign]

Post removed.

Heh heh. I remember what you said though!

[sjs]

Tom, your Tram2 with 45 sounded better than my Tram2 with 2A3, but I have recently had a 26 dht preamp using Llundahl output transformers and that was on another level completely. Still suffers some microphonics mind you

[anubisgrau]

Tom, your Tram2 with 45 sounded better than my Tram2 with 2A3, but I have recently had a 26 dht preamp using Llundahl output transformers and that was on another level completely. Still suffers some microphonics mind you

I'm afraid there is no free lunch with DHT tubes in a small signal stage. Some constructors use a spring suspension of the tube, some go for a very heavy chasis that absorbs fine vibrations that trigger the tube to ring. Some claim a transformer coupling can help but I don't think it's a universal solution. For sure you can add a gain tube before the DHT and then use lots of negative feedback to tame it, but I yet have to hear this option that works just like a straight single DHT only stage.