I went to see my friend Tony today and helped him to fix his 01a preamp implementation. Time ago Tony used a prototype version of my gyrator PCB to build the Gen2 preamp with the addition of an output follower to address the slew rate limitations he had on his system due to the larger capacitive load.
Luckily we found the fault easily and it was a bad solder in one of the smoothing HT chokes. Once fault was rectified, we proceeded to take some measurements of this preamp.
The preamp circuit diagram is below. Is the classic 01a preamp Gen2 with the addition of a basic source follower to drive Tony’s amp:
Looking at the frequency sweep the preamp shows about a gain of x8 (18dB) flat from 3Hz up to about 180kHz. This is impressive and is thanks to the bootstrapped input capacitance of the MOSFET which provides a gentle load to the 01a:
I don’t think you can get better than this 🙂
Here is a plot of the harmonics and THD at a signal level of 2Vrms. Distortion is extremely low and with a double output at 4Vrms increases only to 0.008%:
The build is really neat. The LT supply is on a separate chassis and HT is on same preamp chassis (see TXs on the right). The pair of sockets has rubber suspensions, the Rod Coleman boards are at the bottom and you can see the output caps at the top as well as the volume control and input selector on the left:
Least to say: his system sounds impressive. This is one of the best 01a preamps I’ve heard myself. Well done Tony!
nice build,a little more space tube/resistor for a better heat diffusion
This particular preamp doesn’t generate much heat. In fact, even the Rod Coleman boards with the low filament current don’t need much of heatsink. If you have external power supplies (like I do) you can cram this preamp in a small case without heat dissipations issues.
In my personal experience, this is a delightful preamp which brings a superb detail and tone to all systems I have tried this pre with. A real gem.
Hi Ale,
I know this is an old post but I decided to ask my question here.
With the Rod Coleman regulators, he shows that the filament regulator is connected across the filaments and then the cathode resistor Rk (and bypass cap, if used) from one of the filaments (recommends the +) to ground. With Tony’s schematic above and your pictures of the 01A boards construction the filament regulator ve- goes directly to ground. Which is correct or am I missing something here? Do you adjust the filament voltage through the Rk resistor?
Hi Dan, The bias arrangement in this this preamp is called “filament bias”. Please search around the blog to learn how it works.
Yes. I’ve read all about it from Rod Coleman documents. I bought 12 sets of the regulators from him a while back but he shows a different arrangement with the cathode resistor placement than what is sometimes shown on your website. Sometimes you illustrate the location like Rod and sometimes you have it located differently. I’ll ask Rod.
Is there any gain possible in the buffer stage? It could be useful to have a little gain if driving a single stage output. Does the buffer introduce any colourations? Presumably not from the low distortion and good listening experience.
No, the FET is a follower and if you configure them as amplifier they don’t perform as well as valves in my view, specially swinging larger volts.
They are good for a phono stage input instead. If you need more amplification, perhaps an input transformer in 1:2?
I have an 01A preamp up and running now – not the Gen 2 gyrator, though that will come. For now it uses a Hammond 126C configured as plate choke and a FT-3 output cap. Voltages otherwise the same as the Gen 2. I’m revisiting the 01A after using 4P1L and 26 line stages for quite a while. The first thing that struck me was the family resemblance to the 10Y – same clear, bright sound. Right now it needs a buffer stage as above to drive my twin 4P1L output stage, but signs are hopeful already – making some nice sounds. It’s worth putting some time into optimising the 01A – it repays you with some rather special sound quality.