Russian PSE in Steroids (01a into 4P1L) – Part IV

More power

Our previous west meets east circuit can be improve further. In fact, a compromise made with the filament bias design is that coupling between driver (FET follower) and the output stage wasn’t DC. We want DC coupling to get best performance, to ensure we can drive well the output stage and provide sufficient grid current even when not operating in A2. This can be done with filament bias, however, since we are already introducing a negative supply, I’d prefer removing the filament bias and go for proper grid bias to get best performance of output stage in terms of maximum power and linearity.

The below circuit can be easily implemented with just few modifications from previous version:

What has changed here? Not much, the coupling cap C2 is now between the gyrator and the FET follower. The gate bias resistor R6 provides high impedance to the gyrator load to ensure maximum performance of the 01a driver (minimum distortion given size of load). Not as good as previous version, but good enough. The R6 is connected to a potentiometer which sets the bias voltage. The bias voltage is derived from V2, the -50V negative supply. You can see that this circuit will put more stress into the M1 FET as now there is an additional 25V of drop across it so power burned on this device increases.

The output of the follower is directly coupled (DC) to the output stage. The filament bias resistors are removed and we use the Coleman regulators directly on the filaments of the 4P1L.

This amplifier responds better to the grid current of the output stage once the output power goes over 3.5W. At 4.5W the distortion is just above 3% (3.2%) with a 3Vpp input signal. A tad more and you can get to the 5W and a bit more into A2 operation.

Russian PSE in Steroids (01a into 4P1L) – Part III

From Russia with Love

East-West Divide, Copyright by Justmeans

The interesting combination to explore from our previous designs is to mix some western valves like 01a into the Russian parade.

The result would be quite interesting, as the sound of the 01a has proven to be amazing. Therefore 01a driving 4P1L is possible as the 4P1L doesn’t need a lot of drive. Instead of using 4P1L as a driver, we can opt for the 01a which has a similar gain. What is interesting is that the voltage swing required by 4P1L wouldn’t force the 01a outside the zone in which is highly linear, hence, with some modifications, it can work as a great driver here.

The circuit

Instead of starving the filaments of the 01a, given the voltage swing requirements for a driver, we ought to drive it at full tilt. In the circuit above, the 01a hasn’t got the stones to drive the 4P1L pair, therefore we have added a cathode follower as explained here. The M1 follower will then drive easily the output stage.

Slew Rate in Preamps

Introduction

You may probably already asked yourself: what is this “slew rate” business? Has this guy lost his mind? Probably I have, but not due to this interesting phenomenon. The second question that should have probably popped in your brain is: why? Well, writing about this stuff came up by sheer coincidence.

We tested my friend Tony’s 01a preamp which has an older gyrator board I made for him about 3 or 4 years ago. The preamp (as well as his system) sounds extremely good in my opinion and the 01a has brought a new clarity which is what you’d expect as a result of the introduction of a DHT stage. We decided to run some frequency response tests since Tony has made some interesting mods to his Push-Pull amp. We encountered an unexpected challenge as I didn’t have the right XLR connectors for my testing gear so we run the FR tests on the power amp with the 01a.

The 01a preamp measured as expected with a flat response up until 40kHz before the sound card rolloff kicked in – I’m aware of this and this is the limited bandwidth of my current portable measurement gear. The distortion was also really low (H2 predominantly ) with less than 0.02% for 2 Vrms output signal. This test was done with Pete Millett’s interface which has an input impedance of 100kΩ: Continue reading “Slew Rate in Preamps”

Gyrator PCB prototype

First sets of PCB arrived today. Thanks to Tom Browne for the great PCB work, and it worked first time round. The PCB can accommodate different FETs like 2SK170, BF862 and J310 to suit most of the requirements

If you’re interested in the final PCB let me know, production run shortly due to large number of request for this circuit for DHT preamps and drivers

Ale

Gyrator bias discussion

A very interesting point was raised on the 4P1L DIYAudio thread around the gyrator circuit using CCS and whether a simple resistor divider was better than the CCS due to the LND150 temperature drift.

I’ve tried both options and I’d say I prefer the CCS despite the variation with temperature for the following reasons (which people may well disagree):

It’s true the LND150 varies a lot with temperature (see attached), however if it’s operated at low current (e.g.<500μA) the variation is small. In a cascoded pair for this circuit the drift in the output voltage is small. Simulated in Spice I get about 6.35mV/°C. The resistor divider will be better of course but you need a smaller values to reduce impact of dR/dT. This creates another problem which is the reduced PSR. With a compromise divider to balance idle current and PSR you can get 5 times less variation with temperature in the circuit under discussion – see below (e.g. 1.4mV/°C)
For a smaller value of resistor divider the PSR is impacted and significantly lower than the CCS. If you don’t have a well filtered supply, the PSR benefits of the gyrator will be reduced due to this. For example, I did some quick comparisons by simulating my 01a preamp. I used a 235KΩ/220KΩ and a 23K5Ω/22KΩ divider options with a typical film decoupling cap of 4.7μF.

The PSR of the CCS is above 100dB whilst the PSR of the resistor divider goes from 56dB (235KΩ/220KΩ divider) down to 37dB (23K5Ω/22KΩ divider).
In practice, I implemented two different circuits as I had a shunt regulator before when I had a resistor divider and now I don’t have any shunt regulator but I use the CCS version.

Looking at the output PSR as the gyrator provides additional rejection to noise. The resistor divider PSR is about 73dB and CCS is 30dB better anyway

The voltage variance is really small with temperature and this circuit in particular isn’t affected by such small drift in my view

DHT day

DHT in excess

We met yesterday at Andy Evan’s with our friend Tony for an interesting set of tests. Firstly we looked at measuring Andy’s 4P1L SE and PSE amplifiers:

4P1L SE LL1682: a great sounding amp overall which sounded as well as it measured. A must amplifier to listen to!
4P1L PSE (O’Netics OTs). This one particularly revealed an issue with the 4P1L driver configuration as it was running out of steam at about 2W before distortion creeped in. I think Andy will look into fixing this shortly. It also showed a slight dip above 10kHz up until 20kHz which may be attributed to the O’Netics.
4P1L PSE which I nicknamed the “Daemon” as it nearly screwed up my measurement gear due to some nasty grounding issues. We decided to give up on testing the response of this amp after this.

Andy’s speakers are Mark Audio Alpair 10s full range in some standing 23L cabinets. They do sound great with a solid bass and detailed treble. Perhaps a bit higher on the treble, but they are worth every penny.

We set the listening session to rotate the amps as well as the preamplifiers:

4P1L Siberian Gen 3
Andy’s 26 preamp, filament bias, LL1692 step down transformer and Rod Coleman regulators.
Tony’s 30sp with Rod Coleman regulators and depletion FET CCS loads
01a preamp Gen2

In my opinion, I think we can draw a conclusion to the 4P1L-4P1L-4P1L configuration. Perhaps it’s the H3 harmonic profile, but it doesn’t sound good – a bit harsh on the treble. The challenge in my view was that one 4P1L stage driver wasn’t sufficient to bring out to life in full Andy’s system. It forced the DAC to swing higher output levels and didn’t sound as good as with the preamps.

Don’t get me wrong here, all the preamps sounded great, however there were subtle differences which showed that 01a was superior in this setup. The 26 was also outstanding as expected, however the 30sp was slightly thin. The three agreed on the evaluation carried out and we concluded that 01a-4P1L-4P1L was a wining formula!

The 01a brought a level of clarity that it was superior. The piano, bass, snare drums, brass and voices we listen to in detail across various test tracks sounded with a level of detail and delicacy which was unique. This was a surprise to all, as we were expecting a system with 2 stages to be the superior combination. I wish the 4P1L could have a gain of 20! 🙂

Here are some few pictures of the messy DHTs spread around:

Andy’s HT supplies

4P1L Gen3 preamp, 01a Gen2 preamp, Tony’s 30sp preamp and Andy’s 26 preamp

Andy’s 4P1L SE Amp

What a mess on the floor!

First tests on the 01a, 4P1L preamps into Andy’s 4P1L SE Amp

wiring 01a, Slagle’s AVC and 4P1L Preamp into Andy’s 4P1L SE

Andy’s 4P1L PSE

Andy’s 4P1L PSE a.k.a “The daemon”

Thanks Andy for hosting a great day.

Here are Andy’s impressions posted in DIYAudio of our great experience testing the preamps and the 4P1L SE and PSE amplifiers:

“I had a very interesting day today with Ale Moglia and a friend of ours, Tony. We auditioned 4 line stages. Amp was a PSE 4P1L, with 4P1L input (plate choke and FT-3 coupling cap). Speakers were Mark Audio Alpair 10s. Full range, 23 litre infinite baffles. We placed them in this rank order:
1. Ale’s 01A
2. 26 into LL1692A stepdown
3. Ale’s 4P1L
4. 30sp

All DHTs and all sounded good – quite alike in having that DHT sound. I think they were all filament bias. Ale’s 4P1L line stage might have suffered from going into two more stages of 4P1L so may be better into a different amp. In that sense it may not be a definitive test, so the jury is out on that. We have known from past experiments that 3 4P1L stages in a row just don’t sound that good.

The 26 preamp was predictably nice, sweet and detailed. Audibly better than 30sp. Just a bit better in this context than Ale’s 4P1L but not far away.

The star without any doubt was Ale’s 01A preamp. It was just stunning. Quite magical. So if it’s a question of building a line stage, this to my ears supersedes the 4P1L. I never expected this – I’ve built 01A preamps in the past, but this is a very clever circuit. you can find it here:

01a Preamp (Gen2) | Bartola Valves

I do urge you to look seriously at this design. It’s a bit special. It was the only one of the 4 we auditioned that actually sounded better in the system than without it. There is enough gain with just the 2 4P1L stages in the amplifier, which I usually drive straight out of a ES9023 DAC. But adding this stage was a better sound. I never expect 3 stages to sound better than 2, but this did.”

(Andy Evans)

01a Preamp (Gen2)

A full write up of this preamp design and build can be found here

01a Preamp Gen2 finished!

First build of 2015, just after the New Year. I’m pleased to listen to the 01a preamp gen2 finished. It’s a beauty, only the warm sound of the 01a thoriated tungsten filaments can provide. A perfect match to the Starlight CD player that was lacking of a bit of gain.

It’s dead quiet, the frequency response is superb and the dynamics are there. Very good bass that improves what I was getting out of the discrete DAC. I love listening to this preamp. A great pleasure. Thumbs up to the return of the 01a. My best Sylvania globe valves are now in use, yay!

Will soon write a proper blog entry for this ultimate preamp.

01a Preamp: Build (Part 5)

Finished the preamp build yesterday, still have one power supply to build though. That is the benefit of the Christmas holidays as you have some proper time to work on your projects.

Magically this time all worked fine from start. The preamp measures really well using the noisy bench power supply:

As expected the HF response is really good thanks to the design of this stage. The -3dB pole is above 100kHz. LF response is actually better than the shown above as the soundcard LF pole is playing as well.

From a distortion point of view the preamp is performing well at maximum level which the Starlight discrete DAC will be playing at (i.e. 400mV):

The nice decay of harmonics is characteristic of this fantastic DHT.

01a Preamp Gen2: Build (part 4)

Further progress today with the final top plates for the preamp. Now is time to do what I really enjoy: soldering!