My attempt at designing a discrete mic pre
« on: January 11, 2018, 08:38:34 PM »
Hi everyone, this is my attempt at designing a discrete mic pre based on an opamp topology and Class A output, for the input stage and the current mirror im using the THAT 340 which consists of a matched pair of NPN and a matched pair of PNP transistors in a 14 DIP package, for the rest im using BC212 and BC182.  The simulated THD results are quite good with very little distortion (however the transformer is an ideal one and real results will definitely show higher THD).  Ig uess that the output transistors can be replaced by BD13x type.

The quiescent current is quite high thou, 56mA, the result of biasing the output stage in class A, so its quite a current hog, but im not planning on making a console full of these, maybe a pair and thats it. I got the base design from Self's book and the Nelson Pass article.

All the front end is missing (Pad, phantom, polarity switch, etc.)

Im pretty sure its far from perfect, but I would really like to read your comments

Thanks

« Last Edit: January 11, 2018, 10:09:19 PM by Dualflip »


PRR

Re: My attempt at designing a discrete mic pre
« Reply #1 on: January 12, 2018, 12:07:12 AM »
It can work (sometimes a rarity around here).

BC212AP seems to be (info is slim) a TO92 part rated for 0.3 Watts? Here you have 60mA at 24V which is like 1.5 Watts?

What is the gain/phase relationship? Looks like the middle is rolled-off to unity near 35MHz. That's not huge margin on the BC212's fT of 200MHz. OK, that may work for minimum gain of 10.

I like the Garbage Principle. Each stage of a chain should build on the one before. Little 13g trash cans go in big 40g trash can. Big can goes to 5ton street truck. Street trucks dump to 20ton highway trucks to haul the stuff out of town. At first glance, you have 1mA in first stage, 6mA in middle, 60mA in last stage. BUT you have these puny 0.3mA Darlington Inputs working at low fT. That's like dumping the street-truck into a breadbag, then back into the truck.

And Q10 does not seem to drive Q11's base current, certainly not with authority.

Why the Sziklai at the output?? These things love to sing radio waves. It isn't like PNP and NPN are different prices. (And used to be Si NPN was the cheaper.) A simple Darlington is tamer, makes Q9 and Q11 the same part, saves an order-line, an inventory, and a place for the worker to get mixed-up.

Any unprotected output, I like to ask "what happens if...?". Actually there is some limit of damage, between current-limit Q9 and the cap on the output. Direct-coupled allows more trouble. When things go bad, Q11 melts then Q10 melts. At this point Q7 tries to pull the whole load current. Without CCS and C3 (normal power amp), current is limited only by R15 47r, so could be a half an amp, which Q7 may not like any more than Q11 and Q10 did. Yes, the gig is over in any case. But I like to see "some" limit on how many burnt parts I will have to pull and replace before I can get paid again.

You bias all three stages with one diode-string. The impedance here is low but not zero. There can be sneak interaction across the 3 stages. (Maybe not a lot because none are class B.) I see 2mA in the diodes and near 0.7mA in the three bases, so the diode current is not "much larger" than the loads it is trying to control. I'd pencil 3K at R4 and see if the boss objects to the extra power and heat.

Re: My attempt at designing a discrete mic pre
« Reply #2 on: January 12, 2018, 01:44:12 AM »
Thank you for the indepth analysis PRR, I truly appreaciate your words.

I made some changes following your advice, the Darlington VAS is now running at around 1.8mA instead of 0.3mA, the output stage was replaced and now im using BD136, biased at 27mA, for a total power dissipation of 0.65 W (do you think heatsinks are needed?) I can always raise the current back to 60mA and use TIP41 here and run them with a heatsink, still with 27mA quiescent at the output im almost sure that 600 ohm loads cant be driven to full swing, in simulation this means a peak output of around 15V, not that bad either. The Siklai at the output is replaced by a Darlington stage. R4 is now 3.3k.

Im also including the gain/phase screenshot with its marker at the 180° phase shift, May I ask how did you determined the 35MHz unity gain point analytically?

Regarding the output protection, what kind of circuit do you recommend? should I add a resistor at the output and a transistor with its collector connected to the base of Q9?

Thanks again






« Last Edit: January 12, 2018, 11:34:19 AM by Dualflip »

PRR

Re: My attempt at designing a discrete mic pre
« Reply #3 on: January 12, 2018, 04:06:36 PM »
> how did you determined the 35MHz unity gain point analytically?

R1 R2 C1.

buildafriend

Re: My attempt at designing a discrete mic pre
« Reply #4 on: January 12, 2018, 10:57:29 PM »
Which Douglas Self book did you use? I like his literature a lot, his small signal design book is excellent.

Re: My attempt at designing a discrete mic pre
« Reply #5 on: January 12, 2018, 11:03:17 PM »
Which Douglas Self book did you use? I like his literature a lot, his small signal design book is excellent.

Precisely that one, Small signal audio design, I have all of his books, Douglas Self has a lot of experience and a  talent to explain things, sadly, most of the stuff in his book is "take it as it is" since no equations are provided, I dont know why he avoids math as much as possible, in my opinion it just makes things more difficult or useless, for example his chapter on EQ is great, but no equations are provided so changing basic parameters such as frequency orQ is not an easy task. A lot of the stuff in the book is useless simply because you cant adjust it to fit your needs, other than that, great book, and also unique, I dont know of any other comparable book out there.
« Last Edit: January 12, 2018, 11:07:39 PM by Dualflip »

buildafriend

Re: My attempt at designing a discrete mic pre
« Reply #6 on: January 13, 2018, 09:18:26 AM »
His book Audio Power Amplifier Design might be of interest.  If you want more equations The Art of Electronics has many including a section about op amps.