[Akku]

Hello!
I am trying (just for fun) to reverse the circuit of the headamp I bought on ebay but I have found a component that I have never seen before. It looks like a small axial zener but has a metal case (alluminium color) with a black band on one side (catode? polarization?). On the case I can read a sort of "M" in a circle (Motorola brand?) a "K" and "79 C5 V1 525". There are two for channel connected from imput to ground in the way you can see in the attached sch. I draw them like zeners but it doesn't make much sense to me.
Any guess?
BTW: no, I don't have a digital camera.

 

[TrevorNetwork]

I suspect it is a common diode.

 

[Akku]

Yes, maybe you are right, but what is the purpose to use diodes in that way in the input?

Thak you

 

[rickcr42]

that entire input circuit is an absolute mess !

polarized caps back to back to simulate a non-polarized cap when a simple 10uf non polarwill do.Zener overvoltage protection in a circuit not requiring it,a 50k pot "looking at" a 33K input resistor stting the input impedance..

not hard to clean all that up but all those resistors and no caps for frequency selective shunts ?

not sure what the guy was thinking-maybe i am missing something here............

 

[Akku]

I didn't like the sound of the amp very much so I tried to understand the circuit to see if it was possible to improve. I am not very smart in electronics so I could have done mistakes but I noticed the strange input. I have to double ceck the imput resistor value, 33k for a 50k pot seems very uncommon. The choice of zener protection maybe is because the ps is unregulated

Thanks for the suggestions

 

[rickcr42]

the guy has it all wrong !

The DC blocker ,the back to back electrolytic is a real sound butcher.the zeners are I guess an attempt at input protection,meant to shunt a certain over voltage to ground but is A-not needed here and B-is not implemented correctly.

The capacitor to ground shunt is all wrong ant the size of ther cap means bass frequencies are being shunted.the cap either needs to be WAY reduced in size (around 100pf) or eliminated entirely.It is meant as a high frequency filter but is wrong.

so R10-to 500K,replace the cap with 100pf silver mica or monolithic ceramic,keep the pot even though 10k would be a better choice here (then R10 could be 100K),eliminate all before the pot and then add a 5-10uf film cap in place of the back-to-back electrolytic

i get a chance i will look at the output stage but you need to address the input first.not real hard to do and cheap actually for the amount of sonic return

good luck man

 

[Akku]

Thank you very much Rick, I will try your suggestions as soon as I get the parts needed. I don't want to waste money on that (I am really satisfied of my PPA and I am looking forward the Mcubed) but the mods are really cheap.
The other amazing thing is the unregulated psu. I attach the sch. but it couldn't be more simple. I wonder if there is some cheap and easy way to add some regulation (maybe shunts?).

 

[rickcr42]

bust night man and I just got up so my brain is not yet engaged fully

But after some coffee and a shower i will re-visit the schematic and see if we can do some "on the cheap" power supply mods and maybe if needed clean up the output stage a bit.
The potential is there we just gotta dig it out

 

[tangent]

Quote:

The choice of zener protection maybe is because the ps is unregulated

No, it's just throwing parts at a non-problem.

The lowest zener value is 2.4V as I recall, meaning that that circuit won't break down until about 3Vp, minimum. That's awfully high for a g=5 circuit. The circuit is going to be clipping well before you hit the protection point. About the only thing it will protect you from is a massively wrong input signal. Like, trying to run this amp from the output of a power amp or something stupid like that.

Quote:

I wonder if there is some cheap and easy way to add some regulation

Sure, just drop a 78xx and a 79xx on there. But, with such huge filter capacitance banks for such a small load, I'd be kind of surprised if it gave you an audible improvement. Regulators are nice when huge filter banks are impractical.

 

[jefemeister]

This device is called a TVS - Transient Voltage Supressor. See the following links for some more info.

http://www.protekdevices.com/appnote...20note%202.pdf
http://www.eetkorea.com/ARTICLES/200...13_AMD_AN4.PDF

I've seen it used inbetween chassis ground and electrical ground. Some folks use a small cap for this application instead.

 

[rickcr42]

I agree with jeffymeister that that was the INTENT but the actual implementation is off base

first the OPA has such supression already built in,second the values of the zeners are wrong and finally -totally not required in a low-med gain circuit using an opamp.

If this were a discret jfet design then yes but since the chip already has the jfet protection internall it can safley goo bye bye and good riddence

 

[rickcr42]

OK,should have read all the threads through but did not.What i said is true just not the answer to the question

the power supply-regulations and low noise power supplies are nice but not absolutely required.If you read the earlier thread on "discrete vs IC opamps" one thing i could have added but failed to mention is the PSRR.

Power Supply Rejection Range.This is an area where I.C. opamps beat the pants off a typical all discrete design.

you can do discrete as good but the matching and bypassing not to mention balancing circuits are no easy task or design method and it takes some work.

The good performance in this area means power supply regulation is not essential with modern jfet input opamps for good performance,nice to have but no required.The headwize library has a FAQ page that can answer a lot of opamp design questions in a manner that is easily understood.the strength of the library is the 'everyman" style of writing and level of understanding obtained even by those new to audio electronics.I suggest you check it out just for some bacground info

cheers

 

[Akku]

Thanks again to everybody for helping me.
I'll go for the easy mods to the input stage. Maybe later I will try something more even if there isn't much room in the case: every thing, even two trasfo (one for each channel), is on an eurocard pcb.
I am not really interested in this amp but trying to improve it can be a good way to learn somenthing (I hope).
Obviusly the parts I need cannot be found in the spares I have from PPA and other DIY attempts.

I have to place an order for some resistors and caps.

 

[rickcr42]

how is the parts access in Italy ?

that is one area where I have no links to suppliers in the DIY Links section and if you have a couple of good ones PM me and i will check them out for addition to the list.Would be a great help to others from your country

thanks

rick

 

[guzzler]

I order stuff from AudioKit.it occaisionally and they've always been good to the UK, so I'd imagine they're just as good to Italy

g