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Valvestate 8020 poweramp (LM1875)

Started by Alexius II, June 07, 2011, 05:13:39 PM

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Alexius II

Hi, I need some help/advice.
I am building myself a practice amp. I already have the preamp, but I have yet to build the power amp. I wanted to make something based on LM1875, because I already have the chip and also a 2x15V 30VA toroid transformer (which according to datasheet should give me 15W).

While googling for help about the "mixed mode" feedback configuration I found out that the Marshall Valvestate 8020 features a power amp that seems to be exactly what I was looking for. I decided to borrow that design  8|

Here is the schematic: http://www.drtube.com/schematics/marshall/8020.pdf

What I need the help with is the removal of headphone and line out (D.I.) connections.

Here is my redrawn schematic:

I marked three things I believe I understand: the zobel network, the line out and the remaining headphone out resistors.

I then removed the line out and headphone stuff and got this:
(zobel is drawn very small, so that the main topology is more obvious)


So, I just want to be sure... will this work, or am I doing something wrong?
I've never built anything like it, this mixed mode stuff is all new for me :tu:

EDIT: also, what is the meaning of that 47k and 10uF? I don't see that on the ESP project  ???

phatt

Hi Alexius II,,
What you drew is quite correct.

The reason it is not on ESP is simply because one circuit is inverting the other is not.

Quite frankly this *I* Feedback does little.
I did quite a few experiments trying to see/hear what all the fuss was about but in a room full of musicians all playing together it won't make much diff,,, It does rob some power and on a small Amp it's hardly worth the effort ::)
That is just my observation but others rave about it. Go figure?
Phil.

teemuk

#2
Note that the circuit is an inverting opamp, it will need an input resistor to define gain, and preferably a buffered input to prevent source impedance from interacting with the gain.

See how that Marshall uses the 100K input resistor and a low source impedance OpAmp gain stage as a buffer.

I agree that the whole current feedback scheme doesn't perform any miracles. After all, it's basically just a filter that happens to interact with the used speakers, resulting into gain boost at speaker's resonant frequency and at higher frequencies when inductance begins to notably increase. You can definitely hear something missing if you compare two amps, one with flat response and one with response modified by the speaker load. For some those EQ:ing effects are part of traditional "tube power amp sound". Personally, I don't hear it as a gigantic effect but some people actually do.

People hear things differently and some have greater focus on specific details that other's might not even care about - while they are busy on caring about details the other guy in turn mostly ignores. This is mainly why I don't trust a lot on "ears" nor on other people's opinions on matters that depend 100% on taste. Of some things you really just have to make up your own mind. The current / mixed-mode feedback today is pretty much a standard feature of solid-state guitar amps so to me that gives a more generic view about its usability and importance than my own opinion on what magnitude it alters the overall tone. I don't hear it doing much but designers have been using it since mid 1960's so it obviously must have great importance to many.

Funny thing is, someone even described that effect as "tube sag" some weeks ago during discussion of Rocktron Velocity amps that mimic that non-linear response. Of course it's not sag, but it kinda shows that people do identify some archetypal effects to which they are familiar from tube power amps and may actually give great importance to them, regardless of whether they even know what causes that particular effect. For that guy, the non-linear response was The prominent tube power amp effect, and I got the impression that it pretty much seemed to be a thing that explained all the folklore he might have ever heard about tube (power) amp characteristics. He actually even got quite angry at me when I pointed out he was not hearing anything even remotedly resembling "sag" but plain difference of non-linear and linear response.

Alexius II

Well, I certanly don't expect this feedback to perform miracles, but I am eager to hear the difference. I've played valvestates before, but had nothing else to compare it to. I recently recieved LM1875 pcb from chipamp.com for my hifi amp, so I will finally get to A/B both configurations using the same chip. I also have two tube power amps to compare and hopefully give (a personal of course) an opinion about comparison with those.

So, now my exams are finally over and I have time to continue with this  :tu:

I have a couple of new questions, if someone can help me:

1. It would probably be nice to have a line out. Should I keep the original line out, or is it better to have a buffered line out directly from preamp output? Any benefits to one or the other? Buffered line out could easily have it's own volume control.

2. The last stage of my preamp is a marshall tone stack, after that there will be a jFet buffer, followed by a cap and 10k volume pot. Is this 10k pot a problem, or can I run the line from it straight to the 100k input resistor of the amp?

3. How exactly is the gain of this amp determined? (if I find the need to change it)

4. Do I have to change anything if I decide to use a different transformer? (I suppose the original amp has +/-18VAC, I will use +/-15VAC)

Thanks for all your help! :tu:

Alexius II

My exams are finally over  :loco  :)

So, a couple of changes from my last post.
In the meantime I've finished my Dr.Boogey/TDA2003 amp (another topic) and learned a few more things. First, I made a "line out" as a buffered output right before the power amp and it works ok (two of them actually). I can hook them up to my Mesa 50/50 power amp and 4x12 cab for insane volume and sound. Next, the 3 watts I get from the TDA... way more than enough for bedroom practice (and recording) even at 1x12, but for band practice about 40 or 50W is needed. So, for my use, I don't see a point in building a 20W amp no more, and that means using a LM3886 chip. I will make a new topic for that, as it will probably have a lot of pics, schematics, pcbs. (and I will probably ask for at least a few advices)  :tu: