This sort of follows on from my previous thread on the subject, but between the end of that thread and the start of this one I decided to revert back to a design that follows these specs:
- We use my existing Lundahl 1:32 SUTs as they match perfectly with the DH3S and are in line with manufacturer recommendations
- Valves will do all (or at least 90%) of the active amplification
- Don't be afraid to use solid state for unity gain / buffer stages
- I bought the Sowter 1280 LCR units a little while back so I'm committed to making them work now.
Some time back I found this blog post where the guy had worked around the 600R impedance problem with a step-down and step-up transformer either side of the LCR network. Having pondered this for some time, I've decided that's too many transformers in the signal path for my liking. However my interest was piqued by his choice of input pentode - he used an E83F, which, it turns out, are plentiful and affordable.
Another input pentode came to my attention in the manual for this very expensive LR stage. It turns out they use the Russian 6J11P which has similar numbers to the E280F - these are currently VERY cheap although I do wonder whether the EM/IA manufacturers buy large batches and select from those - at the price being charged that wouldn't surprise me.
Anyway, I've bought a few of both to try out.
Moving on to how we drive a 600R impedance - I've decided to sit a unity-gain op-amp between the first valve and the LCR network to solve that issue, the coupling cap between the two can then be a small value with the op-amp's output directly coupled to the LCR stage. So as a basic block diagram, something like this:
The signal through the op-amp should be around line-level pre-EQ, which is optimal for minimum THD according to page 1 of this datasheet. Given I already have op-amps in my linestage that are inaudible (to my ears at least) to me this seems the most elegant solution to the driving impedance issue.
Post LCR, as you can see from the diagram, I don't know yet. Currently under consideration:
- Plain ole' triode stage followed by either Allen Wright's SLCF circuit or a simple op-amp buffer
- Aikido circuit maybe?