Audionote M7 and Foreplay III

[john65b]

I never realized how close these two designs were until recently. I know the tubes are a bit different - the AN uses a 6072A and the FP is a 12AU7, but the topology looks very close.

I had a FPIII that I liked but had too much gain so I reluctantly sold it. Since then I had a few pres make the rounds (AN M7 with 5687, a AR SP6C, currently a Audible Illusions L1) but always wanted to rebuild the Foreplay

The other day I decided to rebuild my12B4A pre that was noisy as all get out, so I built the AN M7 around the 5963 (6.3VDC version of the 12AU7) - AN M7 is attached. This thing really impresses me. It does everything as good if not better than the Audible Illusions L1, and just might replace it.

Wondering if I should change the 6K2//5K1 resistor with an LED on first triode. The only difference between the FP and the 12AU7 AN M7 is the FP has 22K on anode of first tube and 22k on the cathode of second tube, while the AN has 120K on both...also the FP has the LED while the AN has the two resistors (6K2//5K1)

Anyway, any thoughts?

[john65b]

And the original M7 with the 6072A... Never could find out what the B+ and plate voltage was on the real 6072A.

B+ on my 5693 is right around 250VDC and plate voltage is around 100VDC

[TV Man]

I think the Eastern Electric MiniMax preamp is similar in topology as well. Direct connected cathode follower 12AU7 seems to be a good combination. Any further information on these circuits is appreciated.

[analog_sa]

Just shows you how different people's perceptions are. Never liked anything with a cathode follower, especially with a resistor load. Of course, the PS is often more important for the sound than the active circuit but none is shown...

[TV Man]

Is the B+ 250v in the FP? How many LEDs do they use?

I have a test rig preamp and would be interested to try the circuit out.

Thanks,

[john65b]

Actually if I remember correctly, the FP has 75V/150V on the plates and a B+ of 350V.

Has one Red LED...all from memory...

[planet10]

It is a pretty common topology CC DC coupled to a CF. I have a 6N1P version we diyed. The CF really needs a CCS on the bottom in any of these.

dave

[john65b]

I cobbled together the circuit and as I said, sounded very nice. Noticing the differences in the M7 and FP (and 6SN7 VTV too), and also noticing that I really wasn't pulling much current compared to the 12B4 (The 12B4 pulled quite a bit, but that's per design). So not being able to leave well enough alone, I made some modifications.

I doubled up the 120K resistors and took a quick survey of voltages. I kept the resistance the same on front plate and back cathode (is this important?) Seeing that the overall amp was pulling only 11 ma total (both channels). I then "forced" the front end into more current by replacing the 4K5 resistors (3.2V across) with a red LED (2V across), doubling the current from 1.4ma to 2.8ma.

Anyway, the circuit before and after the LED is below. Comments? I know the 5963 max plate voltage is 250V and I am at 262V...big deal?

[kmtang]

Hi,

I am experimenting using 10M45 CCS for the loading of both amplification and buffer stages. The tube I am using is with 7N7, loctal version of 6SN7. Sounds really nice indeed. The first stage is with 2 red LED bias now. B+ is 245V.


Johnny

[john65b]

I have finally had a chance to listen to this pre as it now stands (lower circuit on above schematic) and it sounds very good. It now has all the sonic attributes the Audible Illusions Modulus L1 I have.

I would still like comments on the circuit ( my current 5963 operating points, etc)

The Power supply is a Triad N68X wired in reverse to get 220V secondary through a FWBR (4 x 1N4007) followed bay a CRCR filter (270uf/450V, 25R, 270uf/450V, 1K) giving a B+ of around 275V with a 12ma loading.