Alpair 10M / FT96H speakers

It’s been a long time since I haven’t tweaked my speakers. After more than 9 years I decided to change the drivers after falling in love with the Alpair 10M from Mark Audio. I listened to my friend Andy’s system (4P1L PSE driving the Alpairs) and decided to get hold of them.

A simple upgrade

As I don’t have much time left for DIY audio these days, I needed a simple solution. I couldn’t build a new set of speakers despite the love I have for some horn-type designs. Bringing new speakers was out of the question, so I had to modify my existing boxes to replace the FE167E. Sadly they didn’t fit straight on, so my friend Tony made me a pair of adapter boards to fit these. Made of MDF I painted them in grey:

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Flexible HT supply

The return of the Jedi

Time has come when breadboarding and HT is not longer an option. With a baby around, I was forced to remove most of the valve equipment. However, the return of the Jedi is closer than you expect. With very little time, albeit at continuous pace when I get a free 30min here or there every weekend, I keep very focused on my objective. The 814 SE monster amp is close to retirement (probably before Christmas) and although I have a fantastic LME amp already working, I need to get the valves back on the scene providing I can prove I’ve complied with “heath and safety” – as my wife well put it.

Well, I had a nice attempt of a wooden frame made out of pine wood. Yes, I’m really bad at woodworking but I’m getting better. The top plates are 4mm thick. I have added a nice set of 2 100mA meters at the front of the frame. A 300VA custom-wounded multi-tap HT transformer from JMS which gives me full flexibility from 100 to 500V in 25V steps. The supply is choke input (LL1638) and filtered with a set of nice 50uF oil caps and 20H chokes. Damper valves used with some nice SiC to form a hybrid rectifier.

Top plates are protected with paper before drilling!

This looks to be quite neat in my view!

The idea is to use this supply for my new 4P1L PSE, 300B and DHT Schade experiments with 6P21S/47 and 1624 valves. A lot planned and so little time for this unfortunately.

Cheers, Ale

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:

The 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.

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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LME49830 Amp build (part I)

Both channels are ready now. Bias set to 750mA instead of 500mA. This is done by adding a parallel 510Ω resistor to RB1 to increase the bias range.

I’m using two pairs of 2SJ201/2SK1530 matched.

Noise is incredibly low and distortion is very low as expected. 10W or more of class A can be achieved from this board with only 24V supply:

Here’s a quick test of one of the channels:

It’s very promising. I need to mount the supplies to the chassis and do a bit of drilling work to get this amp completed now.

LME49830 Amp starts!

Oh yes, a lot of sand! My good friend Tom recommended me to build this amp instead of an F5. Objective is to get baby-proof in 6 months. All HT valves will be removed unfortunately 🙁

300B will be built for a separate room….

(null) — Testing the MOSFET rectifier board!