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Messages - NPreston

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Amplifier Discussion / Re: Power amp transistor choices?? As
« on: May 09, 2021, 10:10:05 AM »
Thanks for the advice!!   I should probably spend more time finding a better heat sink and fine tune the vbe multiplier instead of shopping for more transistors.. only blew one pair of outputs so far...  got my pnp/npn mixed up for instant death.. lol.   Since then I’ve been playing full blast and going a little more deaf every day!! 

Amplifier Discussion / Re: Power amp transistor choices??
« on: May 09, 2021, 07:57:36 AM »
So I believe the devices I am currently using have a ft of 3 MHz.  From reading in the diy audio forums I’m seeing a lot of vas transistors people talking about over 100 MHz and output devices where ft is about 20 or 30 MHz.  Being that these devices signals become negative feedback in the power section was just wondering what audible difference if any this might make in the case of an overdriven guitar signal?

Amplifier Discussion / Power amp transistor choices??
« on: May 08, 2021, 09:22:13 AM »
I’ve just successfully designed and assembled my first highish power amp.  Rails at +/- 45 volts into 16 ohm cab and using up all the headroom.  Currently using tip33/34c for output transistors and tip29/30c in the VAS and drivers.  For the sake of a guitar amp do I have much to gain in using faster transistors I the VAS/driver/ output stages??

Amplifier Discussion / Power amplifier capacitor in NFB question
« on: February 28, 2021, 09:40:34 AM »
Referencing the power amp at  I see C4 is an electrolytic cap.  My understanding is that this cap prevents any DC from being amplified by feeding all DC back to the negative input?  Correct me if I am wrong..   Question is, how can this be an electrolytic if the AC signal swings above and below ground??  I am missing something here..   Also, am I correct in understanding, that together with R7 at 22K, R7&C4 make a high pass filter with the corner F at 0.24Hz??  And overall AC gain would be (R7/R6)+1??

Just trying to understand the nitty gritty of power amps..  every time I think I understand something, I find something else that I dont understand.



Amplifier Discussion / Re: Power supply/ power amp question
« on: January 18, 2021, 10:24:15 AM »

Thanks for the replies.
I agree the voltages I have are nothing to take lightly.  Making a conscious effort to not touch anything with my left hand when working on this and ensuring caps are discharged enough before I go poking around.

when I ordered this transformer a couple years ago, I had expected to get a 36-0-36 supply, but I guess I missed the target on that one..

I've been reading and trying to understand how power amps/inside of opamps work and its starting to make sense..  I will definitely look into for more understanding.

I've hacked together couple power amp circuits (albeit crude ones) at lower voltages with some success.. 
Trying to actually calculate things properly and really understand exactly what is happening where this time around.


Amplifier Discussion / Power supply/ power amp question
« on: January 16, 2021, 09:42:00 AM »
Hi.  I am attempting to build an amp and learning as I go.  So I have built a simple power supply using a Hammond 166m72 transformer which is spec’d with output 72ct which I understand as 36-0-36 at full load..(correct me if I’m wrong). Anyways the supply after filtering with no load is 53-0-53.  Question is when designing the power amp to determine power dissipation of various parts do I use +/-53v in my calculations??  Not sure how much loading to expect once I actually hook something up and apply signals and loads. 

The Newcomer's Forum / What kind of capacitors to use where??
« on: March 03, 2019, 08:16:46 AM »
So I know Electrolytics for power supply filtering, however when I look on Digikey brands like Nichicon have multiple of seiries of Electrolytics to choose from..  all having the capacitance and voltage rating I am looking for..  How does one choose??  Go fot the highest ripple current/lifespan I can find??

Also, when it comes to tonestack/signal path caps, what types are people using?  I've read to avoid ceramics due to temperature instability and being somewhat microphonic.  When I've opened up other amps I've seen a a lot of orange and green caps..  Can somebody let me know what the orange and green caps I have seen in other amps are made of, and/or who makes those kind of caps?  Orange drop caps??  Whats the deal with tropical fish caps..  Should they be something I should look into?

Other than that, it looks like caps can get a little expensive..  So I guess I have some research/math to do.. 

I did remove the rectifier..  3 pins out of 4 appear to be shorted together (about 1.5 ohms)  I suspected  output transistors at first, but was able to isolate them by removing fuses that connect the power supply to the output transistors..  and the short appeared on the power supply side..

I have a Yorkville Bloc250 Bass Amp that I am attempting to fix.  It blows fuses upon power up.  I have determined that the bridge rectifier is shorted out..  I am just curious if anyone has any idea what might cause one of these to go before I simply replace it and maybe cook another one?

I have not replaced the rectifier yet, nor have I attempted to power anything in this amp and look for "warm spots" using a bench supply.  I don't have a schematic on hand to post at the moment (just a paper copy stuck on the amp itself), but one thing I did notice, was from the main supply there are a couple power resistors which I believe feed the Power rails for the preamp.  One of these had a bad solder joint, and the PCB directly under looked a bit darkened suggesting maybe it heated up..   

In any case, being that the pre-amp is mostly op amp stages requiring dual rails, could it be possible that if one power rail was out, that the op amp(s) in the signal chain just max out to one power rail, therby hammering the power amp with DC and therefore asking too much of the Main DC supply??   Just a theory..   

Amplifier Discussion / Power transformer questions
« on: February 16, 2019, 09:26:46 AM »
I'd like to build a power amp somewhere in the range of 30-100 Watts running an 8 ohm cab. When it comes to building a +/- dual rail power supply, I am thinking I will need a center tapped secondary on the transformer.. (or is dual secondary coils better?? if so why?)

Does this transformer seem adequate? Under powered? Overkill??
What would be the maximum wattage amp one could safely build using this transformer?

What models of transformers are others using for their power supplies, and what sort of wattages are they achieving??

I havn't messed around with transformers at all since a few labs 15+ years ago in college..  Know enough to not get myself killed, but don't want to waste a ton of money on the wrong transformers either..

Schematics and Layouts / any tips on reverse engineering??
« on: January 14, 2018, 08:21:25 PM »
HI.  I have a Jordan Bass 50 (which works... but I wanna mess with it) I can not find a schematic for this thing...  started trying to draw it out starting at the input, but found it pretty tedious.  I havn't de-soldered all the various wires going to and from the pcb and fear I will eventually break something turning the board back and forth trying to follow traces and components..  Anyone have any tips or is this an exercise in futility??

Amplifier Discussion / Where are all the big heat sinks?
« on: September 08, 2017, 05:31:27 PM »
So, Iv'e been messing around a bit here and there building some pretty crude amps, and am about to make another parts order soon to replenish the stock of burned out transistors I have went through..   I was just wondering where people are getting their large heatsinks from??  Looking for big aluminum ones with the fins on them..   That being said, I'm sure my "hack" designs have just as much to do with burning transistors as my lack of proper heatsinks..   Anywhere in Canada perhaps?  Not wanting to pay a whole lot in duties ect/shipping ect... 

Amplifier Discussion / Re: Blown TDA 2050 on a Fender Frontman 15G
« on: September 08, 2017, 05:22:01 PM »
Thanks for the info!!  Much appreciated!

Amplifier Discussion / Blown TDA 2050 on a Fender Frontman 15G
« on: September 07, 2017, 09:27:50 PM »
A friend gave me a Fender Frontman 15G to tackle.  Found the TDA2050 is toast.. pulls down the rails to about +/- 2 volts (at least the fuse didn't blow..)  IN any case I've been reading up on using a LM1875 as a replacement as the TDA2050 is no longer made..  Any advice on this as a replacement??  From what I can tell so far it should be a direct replacement..  I don't know the impedance of the speaker as I don't have it, but 1875 datasheet says it can drive down to a 4 ohm load with +/-25 volt rails..  with the old part out the rails measure something like +/-23.8V.     Also, as for heat sinking..  there is a little rubber like insulator on the back of the old part. Would it be wise to try to reuse this on a new part?  Would it need thermal grease?  When searching on Digikey, I can find all sorts of mica and alum/Oxide insulators for TO220 packages, but nothing for TO220-5 which I believe this package is.  Any advice or words of caution??  Thanks.. 

Amplifier Discussion / More differential amplifier questions
« on: April 23, 2017, 05:05:35 PM »
I have been really trying to understand differential pairs lately in the hopes to build a real power amp someday.. I have read through the subject in Art of Electronics as well as looked up some diy discreet transistor op amp examples..  I'm starting to get how it works and tried it out today with mixed results.. It appeared to work for a while, then I tried to add a capacitor to the feedback path to increase AC gain and it started popping transistors..

Was wondering if anyone could help me understand a few resistor values and tell me what I could have set up better??

1)I arbitrarily set R1 at 33k..  not sure how this should be set..
2)I chose R3 for a 2ma constant current source for Q1/Q2.  then assumed 1ma in Q1 and set R2 to 680 for roughly .65 volts to bias Q4. I believe this is how R2 and R3 are chosen??  Also not sure if maybe Im running too much current?
3)I chose R4 based on a value I saw somewhere in an example..   How do you choose a value for this resistor?
4)I believe R5 and R6 can be used to set gain as in an op amp??

Also not in the schematic.. I had a random capacitor from the collector to base of Q4.   

Anyways I hope the attached schematic shows up.. 


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