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Topics - LJ King

Preamps and Effects / 4558 vs TL072
July 31, 2007, 03:03:13 PM

Ok, I'm an op-amp novice, so be gentle.

The Mike Matthews Dirt Road Special's preamplifier is built around the 4558. Two pretty simple non-inverting stages with two feedback paths each - one being a resistor only and the other being "frequency selective" for implementation of the tone and bite control.

I'm thinking I want to replace the 4558 with a TL072. For no other real reason than I have an amp that uses TL072 and it sounds pretty good, and as I understand it, the 4558 is considered a "low quality" device for audio.

I have done some homework. The pin-outs for the 8-DIP package are the same. The critical supply voltage and common mode signals and a whole bunch of other things I don't have a clue about are the same if not better with the TL072.

I can't see any reason why I couldn't just swap a TL072 for a 4558.

Anyone see anything I can't?

Amplifier Discussion / Hexfred and zener
July 15, 2007, 12:01:43 PM

Sounds like two good names for a RPG.

Anyways, is it possible to wire up a hexfred so it behaves like a zener - like for voltage regulation?


I am having a heck of a time trying to confirm (at least in my mind) if this power amp schematic works or not. I want to perfboard build a clone of the entire amp, but I would hate to find out that the schematic is wrong. I've done lots of research over the last two weeks and this is driving me nuts.

Here is the PA schematic section:

The amp is a "Mike Matthews Freedom Amp" produced by Electro-Harmonix during the early-mid 1970s. The single ended B+ supply of 60 volts was either AC derived through a minimal power supply (transformer, bridge rectifier, one electrolytic cap) and/or provided by 40 "D" cell batteries. Advertised as "100 watts peak" its RMS output is somewhere between 15 and 25 watts.

On the schematic, transistors Q5 through Q10 are indicated as either FS37001 or FS37003. I haven't been able to find a datasheet or crossreference for these numbers. However, there is an asterisk by Q9 and Q10 and a note that Q9 is replaced with a 2N6123 for the amps using more than one speaker. Since the 2N6123 is in a TO220 case and rated for 50 watts (with a 2N6126 complement) it makes some sense since they may have to provide more current to drive less than 8 ohms of the single speaker versions. So I'm assuming right now that I can find a suitable substitute using the 2N6123 characteristics.

Let me tell what I think about this circuit before I get to the problem as a mistake in my understanding may have something to do with how I got myself confused if this amp possibly works.

Q11 and Q12 is a complementary Class B output stage. No emitter resistors, but that shouldn't affect it from working. Q9 and Q10 is a complementary Class B driver stage.

Q7 and Q8 had me confused until I realized the only way it could possibly work was push-pull Class A. I assume it provides voltage amplification for the Class B drivers.

Q5 and Q6. Well obviously an input amp. Receives feedback from the output across the emitter resistors through R22 and R24. I think R27 and R28 provide a "centerline" voltage reference (~30 volts) at the junction of the emitter resistors (R21, R23). Q5 and Q6 also have to work as a phase inverter for the push-pull drive to Q7 and Q8.

Up to this point I am somewhat satisfied.

What is causing my problem is Q5 and Q6 - how are they biased? The diodes between the bases don't confuse me, I've seen those too many times like Q7 and Q8. I understand that this will set the bias between the two transistors at three diode voltage drops. I can even convince myself that the need for that third diode is to make up for the voltage drop across the series connected emitter resistors.

What totally confuses me is the lack of a DC path to establish the bias. The input is capacitively coupled. How can the base of Q5 be more positive than it's emitter when there is no source of a positive voltage at its base? How can the base of Q6 be more negative (i.e. less positive) than its emitter for the same reason? I'd be happy if there were a resistor from ground to the base of Q6 and another resistor from the base of Q5 to B+. But there isn't.

One thing that seems odd about the schematic is that the value of Q5s collector resistor isn't the same as the collector resistor of Q6. Everything else is so perfectly symmetrical with this circuit that this seems to stand out like a sore thumb, but I don't know what to make of it other than to assume Q5 needs more voltage gain than Q6 for some reason.

So, either this circuit won't work or I am totally not seeing something. I really hope it's the latter.

Any help?