Tuning the system for ETF

This year I entered the shootout competition and will bring my DHT system to the European Triode Festival in France. It’s comprised of the ER801a stage plus the 01a (if extra gain is needed) and the 4P1L PSE output stage

I will have to swap out the amorphous OPT for the Monolith Magnetic ones as the speaker load is 5R.

It’s going to be interesting!

SiC MOSFET Follower Driver

How many more times

Led Zeppelin wrote a fantastic song on their first album: how many more times. You may not be a rock fan, but hey: what a great song. How many more times do I want to get back to this “slew rate” theme? I don’t know, as much as I have to. Plenty of comments out there of bad designs with wimpy drivers attempting to take the 300B/2A3 or even 45 valves to full tilt with disappointing results. Either way, they always blame the valves.

I came back to revisit the driving of capacitive loads effectively as I’m working on a new 4P1L PSE amplifier. Slowly, but getting there. Previously I looked at adding a buffer to the 01a preamp as a result of slew rate limitations found in Tony’s implementation of this preamp.

buffer

 

 

The circuit design

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Tony’s 01a Preamp

IMG_3291I 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.

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Sweep buffer

I’ve been using my sound card and Pete Millett’s interface for testing. However, the limitation on high frequency response is due to the sound card. A cheap, but yet effective option, is to use the Chinese digital oscilloscope Instrustar ISDS2062B which comes with built-in DDS signal generator with sweep capability. With this little piece of technology, you can sweep up to 10Mhz. The resolution is 12bit, not great for FFT but good enough for a FR analysis. For frequencies above 20kHz, you can use this device to look into things. For FFT and THD analysis, I will keep using the sound card and audio interface as usual.

When I test the device, I found that the DDS output didn’t have the stones to drive loads at HF. Therefore I went back to my workshop and built the following sweep driver. This was based on the great SSM2019 that my friend Mogens sent me:

Sweep buffer based on SSM2019
Sweep buffer based on SSM2019

The circuit is very simple. It’s operated from a pair of 9V batteries. So far, I’ve tested with the 20dB (actually 19dB gain due to 1kΩ resistor at hand) and 0dB gain modes.

The response is good enough for my purposes:

SSM2019 sweep test

I can get 1.5Mhz @ 19dB or 3Mhz @ 0dB gain HF response which is great. I now need to test it again with a real load.

 

Testing the line stage

Introduction

I couldn’t resist the temptation to try and build quickly the SLCF design proposed here.  It was question of building a simple PCB for the tail CCS and the top MOSFET follower. Wiring it then point-to-point could be done in a matter of minutes and a “rat nest” was built fast enough to enjoy this learning experience.

The usual challenges we face when breadboarding circuits

One of the challenges we face when building a cathode follower with a high-gain / transconductance valve is that it can easily oscillate widely into VHF. So we then are a bit more precocious when building the test jig and “try” to have short connections (something which I didn’t do), add some ferrite beads to anode, grid and screen. Also some grid/screen stopper resistors (e.g. 300Ω) are always very useful. If you pay attention to this and check with an oscilloscope with sufficient bandwidth (e.g. 200MHz) you can spot out any nasty oscillation from the valve. I didn’t, thanks to the ferrite beads and stoppers.

The clear challenge of the SLCF is establishing the correct bias point on the top follower due to the high value of the resistor divider and the high-variance we typically get on the VGS(th) of the MOSFETs.

High-value resistors are available on 1% but the variance on the FET defeat the purpose of accurately building the resistor divider.

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A (not that simple) line stage

Thanks to Tim’s suggestion to implement Allen Wright’s Super Linear Cathode Follower (SLCF) which forces the valve to operate in constant voltage (in addition to the constant current) by bootstrapping the cathode to the anode using an extra follower, distortion is reduced further. In my case, it is halved!

The “extra follower” on top of the cathode follower could be another valve. But to avoid elevating heating, I just went for a straight sanded follower using a depletion FET, the famous DN2540. You can use the MOSFET of your choice here:

LME49830 buffer v04The stage gets a tad more complex, albeit not much. We can keep one single supply rail here but we need to elevate HT up to 180V minimum to provide the headroom required by the FET to operate well and minimise its output capacitances to ensure HF response is good. M1 is the top follower which provides a fixed DC voltage using the resistor divider formed by R7 and R8. The higher these are in value the smaller C4 is. C4 provides the bootstrapping between cathode and anode to force constant voltage operation at all times. This minimises the distortion of the valve. I found that 100nF was enough to halve the distortion down to 0.0035% at 2Vrms!

PSR is about 60 to 70dB across the audio band so great addition to have a top active device here!

I think this could be a fantastic follower to use in multiple designs. Worth breadboarding it, hopefully shortly.

 

A simple line stage

Driving your amp

A typical challenge we may all face is how to drive effectively our amp via a stepped attenuator or an AVC. I have a 4P1L preamp which drives very well my AVC, however, I have now an LME amp which has a wimpy input impedance of less than 7K.

How do we deal with this? A simple line stage which is capable of driving the low impedance of the amp is what we need in this case. Several options are available, however I settled down for a simple cathode follower.

Why? Because I love valves, and I wanted to play around a cathode follower design here.

vinilo A heavy load for your preamp or music source may increase distortion and we don’t want that.

I set myself the challenge to design a simple linestage with a minimum number of power supplies. I could have gone for a MOSFET follower, but hey: I wanted some hollow state stuff in there! Ok, if we look into a cathode follower as the core design, this means that we need at least an HT supply and a filament supply. If we could leverage a bucket converter, we could provide the HT from a LT transformer, probably best to look into two windings to separate the filament supply from an HT one. There are cheap ready build step-up converters for peanuts, and this is what tempted me to explore this solution.

I tested recently some step-down bucket converters and was encouraged by the noise levels and the FR.

The first design, getting us started

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LCR Phono: design notes (Part III)

My previous design wasn’t good for two reasons:
  1. Input capacitance was too high due to Miller effect.
  2. Overall gain wasn’t enough: 55dB was marginal as 60dB would be ideal for an MC stage. Obviously this doesn’t apply to an MM cartridge where 40dB should be more than ok.

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AVC finished

Today I soldered the remaining connections to get the second channel up and running. The result is impressive. The AVC project is now finished. And I’m a happy bunny.

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The sound is stunning. As good as I was expecting to be. Very detailed and no loss at bass or treble. The combination of 4P1L driving the AVC and the 814 input step-up transformer seems to work much better this way. The 4P1L preamp has no challenges in driving the AVC, however the full signal swing from the 4P1L step-down via by the AVC seems to sound extremely well.

Thanks to Dave Slagle for such a wonderful AVC. I measured the AVC and matching with a FR sweep and the scope. I was pleased to see the matching between channels and also the nice response 10Hz up to 30-40kHz.

Hard work to wire but it’s worth every penny, trust me…

I have also tried a new set of interconnect cables I made using PTFE shielded RF coaxial. Very simple to build and with nice performance.

The AVC is placed very closed to the power supplies (as you can see on the picture below). The AVC is still very quiet and there is no hum picked up due to the system layout.  Lucky me.

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