audio-talk

I don't want to dump in Mike's (aka Little Eddy) MoFo thread so this is a new one for my project.

I'm thinking of building a MoFo to use for the bass drivers should I progress with a two-way open baffle speaker build, otherwise it'll just be another amplifier to occasionally use. With the OBs I would look to use my 13E1 amp for the mid/top driver(s).

Anyway, the driver stage requirements for the MoFo (which is a power buffer without gain) are an ability to swing 40V; assuming an input to the driver stage (ie output of source component) of 1-2V that means a voltage gain of something like x20 to x40.

My thinking is to make use of a Broskie Aikido Cascade circuit for the driver stage as I have some available from a 6C33C project.

Here's the Aikido Cascade schematic;



and here's the MoFo schematic;

Sorry if the following is noddy stuff but my talent isn't electronic engineering so I'll appreciate any pointers on offer.

Issue #1: the voltage gain of the Aikido Cascade circuit above is x55 - a little too much for my needs. I need to try and understand how the gain is set and make some adjustments.

Issue #2: how to hook the Aikido Cascade stage to the MoFo stage. Can I simply eliminate the 1uF cap and 470K resistor from the output of the Aikido Cascade and just hook it's output to the 'IN' of the MoFo? Otherwise there would seem to be two caps in the circuit between the Aikido and MoFo stages?

I'm now the proud owner of two matching Microwave Oven transformers that will be used for the load chokes for the MoFo modules. They weren't packed as well as I would have liked but they survived intact apart from the corner of one base being a bit bent but it should be simple enough to flatten it out.

I measured the primary windings with my DMM and they are both 2ohms. I've no way of measuring the inductance but I've no reason to disbelieve Mike's (Little Eddy) estimate of around 150mH. I'll need to work out a way of keeping the leads from the secondary windings safely out of harms way.

I found some additional information about the MoFo over on DIY Audio, plus a couple of guys there gave me a steer with respect to the driver stage.

The rule of thumb for the input voltage is that it should be roughly the same as for the power supply voltage for the MoFo boards, so if I assume +20V for the power I will need a max voltage of around 20V on the input. So assuming my DA outputs 2V I'll need a voltage gain of x10 in the driver stage. In light of that I'm going down the route of using Broskie's CCDA circuit for the driver stage, using a 6SN7 configured for 20dB (x10) gain.

13E1 first then I'll get stuck in.

Ray P wrote: ↑Tue Mar 26, 2019 11:05 pm In light of that I'm going down the route of using Broskie's CCDA circuit for the driver stage, using a 6SN7 configured for 20dB (x10) gain.

When I was making my Snail phono/line stage, Nick suggested something very similar for the latter but with matching CCDs in both gain and cathode follower stages, in my case both ECC82s running at 5mA. Doing the same with the 6SN7 running at around 10mA shouldn't be a problem, and the CCD is a 2-wire device so they could simply be connected to the pcb, replacing the two load resistors. I used cascaded two 10M45s but if I were doing again would probably just use single devices. That assumes you're OK with CCDs, but you may have a sonic preference for resistive loads.

Thanks Mike. Pretty intense few days at work at the moment so no time for much else but I'll try and round up my thinking on this build in the next week or two.

So for connecting the CCDA and MoFo, referencing the schematics a few posts up, I'm going to eliminate the 470K load resistor from the CCDA and link the CCDA's output cap (1uF) to the MoFo 'IN'. On the MoFo I'll replace C1 with a link. If I can find time I'll sketch out a schematic.

For the MoFo power supply I'm going to adopt the approach used by DIY Audio member xrk971 and use SMPS modules feeding cap-multiplier circuits, just the +ve part of this one;

https://www.diyaudio.com/forums/power-s ... plier.html

A pair of DIY Audio MoFo PCBs arrived yesterday.

This talk of 4P1Ls on the Nemesis amp thread has got me thinking about whether it might be a good driver for a MoFo. Some research required.

Ray P wrote: ↑Mon Apr 08, 2019 8:04 am This talk of 4P1Ls on the Nemesis amp thread has got me thinking about whether it might be a good driver for a MoFo. Some research required.

Ale has done a lot of work with the 4P1L http://www.bartola.co.uk/valves/

He did a talk at ETF on getting the best out of these types of valve. The notes of that are on his site some place. And there's a ton of stuff on DIYAudio also but pretty much summed up by Ale on his site.

Cheers,
Stephen

Yes, Ale's pages were my starting point - I wish I hadn't sold on the gyrator boards now!

This looks like a good starting proposition;

http://www.bartola.co.uk/valves/2017/06 ... amplifier/

I've got two of Andrew's DHT filament supplies available (the 'large' version) and I'm wondering they would be a swap in for the Coleman regs - don't see why not.

I seen to remember that filament bias was supposed to be good ?

I did have 4 of them myself, but I sold them a year or so ago.

I feel a bit silly having ditched my original now. It would have been good to compare my opamp front end(ltc1150) with one of the valve front ends that seem to be appearing. The original had great promise from the first sounds that came out, but alas It needed bigger chokes and much bigger sinks.

It might have revitalised that old chestnut of what it is we're actually hearing, the front end or the back end, and which makes the biggest difference...I just loved the discussions that revolved around that one.

I've sent an enquiry to Ale about getting hold of a couple of his gyrator boards for a 4P1L driver stage.

I need to study Rod Coleman's notes on using his filament supplies for a filament bias arrangement with 4P1L valves to see if Andrew's DHT supplies are suitable for the same arrangement. The application notes are at the bottom of this page;

http://lyrima.co.uk/heatv7pdf/