@frankwilker -- you use a DVM ammeter to set the anode current to 20mAby adjusting the parallel resistor. I use a 10k multi-turn potentiometer and adjust until satisfied, then measure and implant a parallel resistor OR calculate the appropriate value for one resistor.

Hello

so, Im having fun and games playing around with the loading.

Ive tried using a balanced sound card but the output has its own ringing so thats not worked too well. Currently using a cheapo usb powered signal generator that does produce a pretty good square wave. Im running that it to the 30k 12R (to match my cart) 30k resistor network to knock the level down. This seems to be working reasonably well.

However, Im finding that higher value of resistors arent damping the ringing enough and i am finding i have to use much higher value caps to stop the ringing and this slows down the rise time too much.

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I can get really good square waves with lower value resistors but Im finding its knocking the output of the source down quite a bit more than is ideal. Down 50% where i get the best output shape.

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Im also noticing that the more capacitance i put on the load it distorts the input across the 12R resistor. Ive put a scope channel across the source resistor to see whats going on there.

Currently my best compromise seems to be 3K3 with a 2n2F giving me a rise time of 4us and a 2.3db drop. Which is about 23%. But theres still a little bit of overshoot.

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This however loads my cart at 33ohm using 1:10 windings, way lower that the recommended 100R from the benz website.

Anybody got any thoughts on how i choose where to compromise?

Also, does that distorting on the source matter if the output is whats its meant to be? At some loadings there isn't that drop, it just overshoots. Im not surprised that its very interactive but not sure of the effect on cart regarding back emf and goodness knows what else.

Ah, looks like i may be doing this back to front.

I assumed that the transformer loading was done first by its self, outside of the circuit.

So should i build the rest of the preamp and load the transformer in situ to give a super square wave on the output of the preamp or am i still measuring the output across the load resistor but with a powered D3a in place?

Ive just looked at the signal across the 12r resistor in my cartridge simulator without the transformer attached and im getting a 1 mV square wave with a pretty clean 80ns rise time, very little overshoot.

I had the transformer hooked up to that and was measuring across the load resistor/cap on the output of the transformer.

I spent a couple of days playing with the relevant transformer formulas and measuring/comparing output levels from different resistors and capacitors and checking the rise times and bandwidths etc. The measurements reflected the formulas surprisingly closely with regards to attenuation at lower loading values.

I had assumed that the load resistor was dominant in setting the input resistance of that part of the circuit and that the input capacitance of the D3a was going to around 200pF all in. A comment in the article about input capacitance of the D3a convienlty matching what was put on the transformer during load testing lead me to believe Sy was loading his transformer separatly to the circuit.

With regards to using a 1meg with 10k pot over it, I feel ive already essentialy done this the long way with individual resistors and have taken a good couple of dozen pics of the transformer inputs/outputs on the scope and now have a fairly good feel of what the resistance and capacitance do to the signal.

However, the formulas also suggest i should be using a minimum of 12k to load with or my cart would be seeing less than 100ohm reflected load.

Ive just had a closer look at the band width.
With the 3300R and 2n2F a sinewave starts to attentuate around 45khz
If i drop the cap to 100pF then its going up to around 200KHz before starts to tail off.

The problem im having is that if i use a large enough resistor to avoid attenuation of the cartridge, the value cap I'm having to use to damp the rest of the ringing is getting large enough to slow the rise time over 4uS.

If i hook up to a powered D3a is this likely to change signaificantly?

Thanks

I'm thinking of using an ultra low noise preamp 1.1nV/hz opa1642 at x20 for the input.
This could be mounted in the turntable for MC cartridges.
then feed into 6922 to perform RIAA.
best of all worlds as noise levels are SS best and RIAA eq is performed by a tube that can easily drive 10K 300pf cable loads.