Tag: Rod Coleman Regulator

4-65a SE Amp: 46 Driver Gyrator

 A day of PCB etching

20121230-194358.jpgAfter a lot of work today in designing many PCBs, I finally got a pair of mu-follower MOSFET gyrators for the 46 driver stage.  The driver has to provide very low impedance to operate the 4-65a output valve in class A2. The gyrator in mu-follower configuration will enable the right bias point as the amplifier is DC coupled as well as maximum signal (and current in A2) with minimum distortion.

Many don’t like sand at all in their amplifiers. I have a lot of experience with gyrators and CCS loads in pre-amps and drivers as well. I have to say that with MOSFETs gyrators the sound is really nice. For an A2 driver, not many options are available and the gyrator is a great choice for this job.

I built two PCBs (one per channel) and the circuit is the classic depletion-mode MOSFET gyrator based on the high-voltage IXTP01N100D. I guess that a DN2540 should work as well here but I’ve been saving the IXYS for this occasion. The reference voltage for the anode bias point is provided by the CCS formed by M1 (LND150) which provides a higher impedance in AC improving the frequency response of the gyrator.

4-64-65a driverThe 46 is operating in triode-mode and filament bias with a Rod Coleman filament regulator. R6 is approximately 1/gm and output voltage is set by P1 to achieve the 4-65a bias point as the amplifier has stacked power supplies given coupling is DC, so no capacitors in the path to the grid.

Next: some tests on these gyrators and the filament boards…

 

 

 

Improving the CX301a DHT pre-amp

An idea discussed with Rod Coleman, Andy Evans and others in DYIAudio forum which still is evolving. The purpose is to increase the output driving capability of this preamp using a cathode-follower stage. A bonus is to provide the filament bias using the cathode-follower valve filaments:

CX301a DHT preamp cathode follower

A 26 DHT pre-amp from Albert

Albert sent me recently some great pictures and feedback of his 26 pre-amplifier using gyrator as the anode load similar to my design on the 26/01a.

DSCN0734 DSCN0742Here are Albert’s notes on his 26 preamp:

“I have my 326 with separated power supply, 5U4G – 8uf PIO – 20H – 50 ASC – 30H – 50 ASC + 18uf pp feeding the  Salas’ HV Shunt V2 Regulator, then to 2 ( left and right ) cascoded DN2540 gyrators. Rod’s filament supply coupled with filament bias @ 850mA with anode @135v. I don’t have anything to check the distortion, but this is the most good sound preamp that I have finished to date. It pleases my ears so much that I have the intention to turn it on once I’m in the house. It sounds gorgerous with very good image seperation. Sound stages are wide, deep and absolute quiet. The highs are sparkling smooth with lots of details. Mids are lush and warm with admirable vocals. I’ve found it even better at the lows if compare with 2x 156c. It goes deeper with clear notes.”

 

 

A new 4P1L Siberian from Martin

Martin kindly sent me a couple of pictures of his latest DHT preamp incarnation of the 4P1L Siberian:

Here are some notes from Martin about his pre-amp:
“A bit more about the circuit, I have built the power supply around the nice AZ1 mesh rectifier. First cap after the rectifier is an obligato 6,8 UF oil followed by a 10H choke and a 100 UF ASC followed by the SSHV2 that I purchased via DiyAudio group buy. On your suggestion I have used the CCS from your 301A preamp. If I refer to my previous preamp with the EF6 penthode in triode connection in parafeed configuration I can say that the 4P1L preamp sounds more detailed and with more air around the instruments. Finally I will do some tests with different output caps to find out if this makes a difference. At the moment I use the Vitamin Q.

Best regards,
Martin”

Testing a CX226 pair

A nice pair of CX226 in the new preamp. They are in very good shape, very low distortion measured in the workshop time ago and no hum induced, so great for this preamp.

Sound is definitely warmer, without loosing any of the bass and bright treble from this preamp. A great preamp so far, happy like a bunny!

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New modular preamp 26/01a and 4P1L

Today made some progress in building the modular pre-amp using my latest design on the 26. I’m planning to make this preamp a modular one to enable quick changes and replace pair of valves and OT. This will enable to use 01a and 4P1L as well…

I used a floor plank I found around as the base. The front is a simple aluminium piece that holds the stepped attenuator. Both 4-pin sockets are standing with the rubber suspensors that DHTRob kindly sent me. The two resistor arrays for filament bias are placed between the valves and the OPTs.

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There is plenty still to be done: connectors, filament regulators and HT bits.  Once all drilling is done, I will then move onto wiring…

 

26 DHT preamp with LL2745 OPT

Today I managed to finally test the Lundahl LL2745/8mA specially designed by Lundahl for Thomas Mayer for a 26/01a preamp / line stage.

The circuit tested is here.

Initially did some tests with fixed bias, normal DC heater supply from my workbench and HT from a passive capacitor multiplier also available in my workbench. Circuit breadboarded has long cables and we shouldn’t expect good 50/100Hz noise levels as have many transformers and things around 🙂

Test 1: fixed bias

  1. Vgk=-6,8V
  2. Ia=6mA, Va= 119V
  3. THD=0.033% @ Vo=1.4Vrms

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Test 2: fixed bias with Rod Coleman regulator

  1. Vgk=-6,8V
  2. Ia=6.5mA, Va= 119V
  3. THD=0.031% @ Vo=1.4Vrms
  4. Vf=1.4V (minor starvation)

Test 3: Filament bias with Rod Coleman regulator

  1. Vgk=-6,3V
  2. Ia=6.5mA, Va= 119V
  3. THD=0.031% @ Vo=1.4Vrms
  4. Rfilament=5Ω, Vf=1.4V (minor starvation)

As we shown earlier in other tests, filament starvation reduces THD slightly as expected. The OPT performs really well. Probably will look at starving a bit more filaments whilst doing a listening test

Will now proceed to rebuild the 26 DHT preamp with LL1660 with this circuit 🙂

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4-65a SE driver

46 DHT in triode mode as a driver

46 driver THD analysis

Continuing with the design of the 4-65A SE amplifier based on M. Koster design, I’m in the process of tweaking the 46 driver to optimise the operating point and provide maximum distortion to drive the demanding 4-65a. Here is the circuit I’m currently working on.

The current 46 driver will be biased at around 25-30mA using filament bias, so Vgk will be around -16 to -17.5V using a 10Ω filament bias resistor array. This will set the 46 at around 185-210V which will give sufficient headroom (i.e. need about 200Vpp max) to drive the 4-65a.

So today I look at varying slightly both anode currents and Vgk to see impact on THD.

So minimum THD is around -17V and Ia=30mA. So if setting the Rod Coleman filament regulator to ensure that Vgk=-17V and the anode gyrator to set anode voltage to ensure Ia is close to bias current would provide the minimum distortion (which is 0.04% in this example). Pa is close to 7W, but looking at the datasheet we can see that maximum Pa is 10W (as the latter 45 version).

46 Super Silvertone THD

 

So next I need to build a prototype of this driver with filament regulator and gyrator load.