Pre amp for Power follower

[ervinl]

Hi, I just try Pavel Macura Power Follower. Since the gain is only < 1, I wonder how to select the pre amp design.

Need advise how the characteristic of sound if I use the following pre amp:
- Simple 1 transistor like mic pre amp, common source, R collector around 1 k-10 k, R bias around 100 k, from basis to collector (for feedback)

- Similar to above, but the current source is from active CCS (using 1 transistor more) instead of static R for bias

- Using SRPP design like tube using FET or BJT

- Using FET, common source like Pass, but with higher R (lower current), e.g. 100 Ohm or more.

- Simple Op Amp (like 4558)

Thanks,

Ervin L

 

[juma]

Read this page (figure 2):
Project 83 - MOSFET Power Follower
It's a nice preamp and it will connect to power follower without coupling cap.

 

[Gordy]

Ervin, for now do not think so much about 'character of sound' and instead consider technical requirements. Once you have something built and operating reliably you can then tweak and change for sound characteristics.

The first consideration is gain / amplification. To get max from the PM Follower needs about 10V RMS into the follower. If your source is a CD player then it has about 2V RMS output, so you need a gain of about x5 in your preamplifier.

There are many ways to do this. Juma has suggested one way. Another would be the Pass DIY preamps such as The Bride Of Zen. Or (if you want something unusual) a Pass B1 buffer feeding into a step-up transformer which then feeds your output follower!

 

[ervinl]

Read this page (figure 2):
Project 83 - MOSFET Power Follower
It's a nice preamp and it will connect to power follower without coupling cap.

I try this pre amp but still fail. R9L is blown and Q2L is shorted. I don't use Q4L but I use double dioda 1N4148 in series as voltage regulator for this CCS. I also connect R7L to center instead of GN (I use + CT - for this pre amp), and connect feedback by pass (C2) to CT also. The input ground is also connected into CT to get it balance (hopefully).

I do not use BC 549, but 546 instead. I don't use BC 559 but 558 (I know it does not complement, but its not push pull right?).

Do I have something wrong connecting to the CT, or due to the spec of BC 558 is to low in voltage (30 volt, while I use + and - 20 V or 40Vpp).

I want to bypass coupling caps using CT.

 

[PMA]

You might like to try this one for the power follower:

dispre

This is the no global feedback version.

 

[ervinl]

You might like to try this one for the power follower:

dispre

This is the no global feedback version.

Hi Pavel,

Wow, the circuit seems complicated to me. Anyway, how to avoid hum with this Power Follower of Pavel Macura. Power itself doesn't have problem, but if I connect with pre-amp in the same box, same supply, than there is a hum although I try to serie the source with 1 K R to source of pre amp.

What is best to connect the source signal ground and output speaker ground, to V- or CT? I have 22-0-22 supply, where +22 goes to 40 in the circuit, and -22 goest to GND in the original circuit. I use 10A CT trafo (18-0-18) with 2x 10000uF/50 Elco (Panasonic).

Back to pre amp, is it still good to use only simple pre amp with 1 transistor vs multi transistor (refer to keep it simple/simple is the best to avoid any unwanted coloration).

Or what if I use small GC (like 1875) as buffer/low gain pre amp for this PoA? Is it recommended or not?

Thanks,

Ervin L
-- still fail to clone your original pre-amp side originated from Red Elliot --

 

[PMA]

You probably have a wiring/loop problem. It is almost impossible to give a long distance advice. Drawing of all the cables and wires would be needed.

If wired properly, this amp does not hum, even with preamp in the same box.

 

[ervinl]

Thanks Pavel. I just realized when opening the datasheet, that the transistor pin out is CBE, not EBC that I assume when soldering the first time. All transistors weere replaced since they should be defect, and it sounds. First I still see a hum, and trace where it comes from.

I suspect it comes from C coupling of feedback bypass. In your original schema, it bypass to V- (or GND). Since I use dual/balance supply and connect all signal ground to this CT/0 of transformator, I suspect that this should go to CT (0) also. And it works, hum disappears.

Anyway, by doing so, I'm confious of polarity, since it can be reversed due to bias/offset adjustment, and by the signal itself. Should I use non polar cap, or is it just safe to use bipolar for low signal?

The speaker (output) itself can bypass the Elco due to offset adjustment. But if MOSFET was broken, the speaker may be gone fast. If I protect using Elco (still using Elco), then how is the polarity, since it should be non polar (since it can swing + to - relative to CT/0 based on signal). Should I use Elco with polarity reversed each other (- + + -), but how about the quality? Or should I just add resistor in series (low watt) as protector? If so, what R and wattage value recommended?

Another questions, IRFP 350 (original) may be hard to find. Most common one is IRF series like IRF640 I use now (TO220), but it can not transfer its heat effectively, so it may become shorted (may be as internal security) when going hot.

Another MOSFET available (just found) are: 2SK 3911 and 1058. Not sure about IRFP 240 (not checked yet). What is the recommended MOSFET that has good audio quality other than IRFP 350? Another could be MJ 150**, Mexico and Singapore (ST) one, not remember the type (the local seller said).

Thanks,

Ervin L

 

[kenpeter]

Step up transformer?