London Power/Kevin O’Connor Master Volume Circuit

[syscokid]

I’m trying to find more info on the LP MV circuit. Has anyone here actually tried it? Many years ago I bought the LP MV kit, which comes with a dual 1 Meg audio pot, two 10K 1/2 watt metal film resistors, and a document page that has notes and schematic of the MV circuit.

I’m planning on trying the new MV on one of my 2204’s. In fact, what I really want to do is to replace the stock pre phase inverter MV with the LP MV. I’m submitting the LP document and a couple of mockups of the 2204 schematic and its layout for y’all to see if I got the wiring correct…:

LondonPower MV Mod.jpg

JCM800 Schematic LP MV Mod.jpg

London Power MV Mod.png

[LOUDthud]

Interesting circuit in that it turns down the feedback at the same time it's turning down the signal from the preamp. I'll have to try it next time I get a chance.

[martin manning]

...submitting the LP document and a couple of mockups of the 2204 schematic and its layout for y’all to see if I got the wiring correct…:

Schematic matches the LP doc. Looking at your layout drawing I'm not sure how you are planning to do the shielded cable. I would run 2x 2-conductor shielded cable from the board to the dual pot, with the shields connected from the CCW lug of the pots to the to the junction of the three resistors as shown in the LP hybrid layout doc, and then run one unshielded lead over to each 10k stopper soldered on the socket pins. Make sure that the pot housing has a good chassis ground by using a toothed washer on the bushing.

[Masco]

A.K.A. The bootstrap MV.

[Helmholtz]

Interesting circuit in that it turns down the feedback at the same time it's turning down the signal from the preamp.

Yes, but the same effect happens with any post-PI MV, because the NFB signal varies with forward gain.

[LOUDthud]

I wouldn't expect changing the Open Loop Gain and changing the Closed Loop Gain to sound the same. If one side of the pot was used as a Cross Line Master, you could change Both OLG and CLG with one control.

[syscokid]

...submitting the LP document and a couple of mockups of the 2204 schematic and its layout for y’all to see if I got the wiring correct…:

Schematic matches the LP doc. Looking at your layout drawing I'm not sure how you are planning to do the shielded cable. I would run 2x 2-conductor shielded cable from the board to the dual pot, with the shields connected from the CCW lug of the pots to the to the junction of the three resistors as shown in the LP hybrid layout doc, and then run one unshielded lead over to each 10k stopper soldered on the socket pins. Make sure that the pot housing has a good chassis ground by using a toothed washer on the bushing.

Thanks. The shielded wire that I tried to illustrate is a 2-conductor setup. The grey colored wire connecting from the CCW lug to the junction of those 4 resistors is the shielding wire. Right where you see the red and grey wires coming out of the cabling jacket is where I’ll make a small incision and fish-out the red wire and just enough grey wire to be able to tap it with another wire to complete the connection. This will allow for the shielding effect of the white wire to continue all the way up to the 10K grid stopper resistor.

[syscokid]

A.K.A. The bootstrap MV.

Thanks for this info too. I just got through researching “bootstrap MV”, and it led me to bunch of other info links like this one:
https://el34world.com/Forum/index.php?topic=21173.0
Looks like the OP at the above thread link came to the conclusion that the two 1MEG grid leak resistors need to be removed!

[Helmholtz]

I wouldn't expect changing the Open Loop Gain and changing the Closed Loop Gain to sound the same.

Both the LP MV and a PPIMV (being within the NFB loop) change the amount of NFB as well as closed loop gain.
As a result the speaker damping decreases with lower MV setting.

The 10k grid stoppers help to avoid grid current effects like blocking distortion.

[pdf64]

Doesn't the bootstrapping effect cause the taper to be weird, eg such that a log pot will effectively have a steeper more linear taper?

[Helmholtz]

Doesn't the bootstrapping effect cause the taper to be weird, eg such that a log pot will effectively have a steeper more linear taper?

I might be missing something, but I don't think the position of the wiper affects the taper.
The bootstrapping doesn't actually change the resistance of the pot, rather by applying an in phase signal voltage to the lower end of the pot or resistor it lowers the signal across the resistance (compared to connecting it to ground).
Lower voltage means less input current. Less input current makes the input look higher impedance.

[John_P_WI]

As others have said, this is the bootstrapped master in his TUT books. The advantage that it has is that it is simple and doesn't affect bias circuits. I played around with it 20+ years ago when I was building to use with his power scaling circuits. Don't be afraid to use a much smaller pot, say 250k since it is bootstrapped.

[Helmholtz]

The bootstrapping with a LTPI is not huge.
As the cathode signal must be 50% of the input signal for balanced operation, the input impedance is just doubled.

[syscokid]

Don't be afraid to use a much smaller pot, say 250k since it is bootstrapped.

Would the taper change with a different pot value in this circuit?

[pdf64]

Doesn't the bootstrapping effect cause the taper to be weird, eg such that a log pot will effectively have a steeper more linear taper?

I might be missing something, but I don't think the position of the wiper affects the taper.
The bootstrapping doesn't actually change the resistance of the pot, rather by applying an in phase signal voltage to the lower end of the pot or resistor it lowers the signal across the resistance (compared to connecting it to ground).
Lower voltage means less input current. Less input current makes the input look higher impedance.

https://en.m.wikipedia.org/wiki/Voltage ... vider2.svg
My understanding is -
if R1 and R2 represent a 1M pot set 50% rotation, R1 will be 900k, R2 100k.
The Vout will be 0.1Vin.
Bootstrapping between Vout and common will increase the effective value of R2.
eg if the bootstrapping doubles the R2 in circuit effective resistance, then
Vout = 0.2Vin.
Hence bootstrapping a pot will tend to alter its taper.