Designing for guys who like designing

[Sch3mat1c]

This time I'm not going to be so easy on you guys, I'd like to hear theory of operation from you

This will drive a zero-bias 6V6 PSE (160Vp, 150Vs). Should sound good if the curves are any indication. Feedback comes from the plate rather than speaker output, since I want the I/O isolated to protect anything from the line-operated power supply.

The goal for bias is Vo=0V. Speaking of which, the diode and resistor are represenative of the grid (25mA at 15V), not actual parts.

Tim

[dhaen]

Thanks for bringing us this morsel Tim,

Ingenious.
Does the 220K serve any purpose? It would be surprising if it didn't in a minimalistic circuit like this.

Cheers,

[analog_sa]

I can see no possible purpose for the 220k. Not that i am not baffled by the whole

[SY]

Ah, the proverbial singing pig.

The 220K, I'm guessing, is used as a startup means of getting bias.

[Sch3mat1c]

You're welcome

The concept with the 220k was to stabilize DC bias. If the output rises say 1V, the rise forms a voltage divider with the bias pot, putting approx. 1/3V on the input. This is inverted and amplified to about -10V at the plate, and maybe -8V after the CF (I haven't calculated gain and distortion for the CF yet), more than countering the original +1V error. That's near what, 20dB of DC NFB?

I could just as well go without it, but I figure it will help stability in all conditions (startup, etc.). Or I could go cap coupled between the input, preamp and CF, but that's not as interesting

Tim

[analog_sa]

It appears the DC NFB will work just as well without the 220k

[Paul Barker]

Hi,

what is the measured dc potential at the input grid? I concerned that the input transformer may short especially if bifilar wound.

Otherwise it's a neat way to achieve two direct coupled stages from a single plus/minus supply, rather than the more easy method of direct coupled cf drive using two different supplies for each of these stages and cap coupling the common cathode to the cathode follower.

[analog_sa]

Why would there be a problem with the input transformer? You can't seriously suggest the grid current could kill it. And there seem to be 3, not 2 dc coupled stages.

[dhaen]

I see the 220K is an integral part of the NFB circuit.
The input transformer's secondary is at the 6V6 control grid voltage. There is no reason why the primary shouldn't be earthed. (Except CMMR).

Cheers,

[Paul Barker]

Hugh? 3 direct coupled stages? Do you count the input transformer as a direct coupled stage or is there a stage I missed?

With regard to shorting input transformer, I blew up a Sowter TA that way, just advising caution and check with transformer supplier. Brian Sowter blamed me for the problem and I had to pay for another transformer, his are bifilar wound and cannot take any dc offset.