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Old 10th September 2014, 01:28 PM   #1
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Default Help needen on a power buffer

Dear forum members, my name is Bas (pronounce as Buzz) and I'm from Holland. I've been a regular Diyaudio visitor lately, mainly because I'm looking for a high power discrete current amplifier.

I'm a guitar player and electronics enthusiast. So far I've build 3 out of 4 tube guitar amps I'm currently using. Most of them are non master volume models. Anyone familiar with older Marshalls and other non MV models knows you have to crank them to get the power tubes to overdrive. Why would you do that you might ask. Well, because driving the power tubes into saturation is one of the key ingredients of the best known hard rock sounds. Think of ACDC, Zep, etc. Really, there is nothing like a cranked Marshall.

However, a cranked 100 Watter through highly efficient is extremely loud. Window shattering loud! That is why lots of guitarist attenuate their amps. You get manageble volume levels and power tube saturation.

I've got an Ultimate attenuator (UA). It works simple: the amps output is attenuated by a 30 Ohm resistor and the reamped by a simple class ab buffer amp. There are a couple of places where UA cut corners. 1 The load is not reactive, thus it's not a realistic load to the guitar amp in front of it. This makes the amp sound flat (lacking highs and lows) 2 The load is to high: 30 Ohms on the 16 Ohm tap could be a problem. 3 The reamping is done by a very basic class ab buffer with a pair of 2sa1943/2sc5200. Very poor damping factor and distortion character.

What I want is a load that is realistic to the guitar amp, thus a reactive load reamped by an amp that is pretty linear and doesn't interact with the speakers that much. Seems more logical to me. I'm going to build this and some parts I've already gathered.

Designing the reactive load was a peace of cake. Randall Aiken made a great tutorial but without the reamping part. I've heard sound samples of reactive loads amplified via a line out from the reactive load and I thought it sounded great. Much better than the UA at this moment. I'm using this load to simulate the buffer in Ltspice. Schematics are below.

The reamping part is giving me a serious headache, as I'm just beginning with designing SS amps. It has to be a buffer (unity gain) with good damping and low distortion. This is what I've come up with and where I need suggestions.
Click the image to open in full size.
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Last edited by Rootz; 10th September 2014 at 02:15 PM. Reason: adding pictures
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Old 10th September 2014, 02:42 PM   #2
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Here is the schematic for the original Ultimate Attenuator.
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Old 10th September 2014, 03:42 PM   #3
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It helps if you're practically deaf.

You're not the first person to do this. It is typically done with a "practice amp" - a small amplifier and speaker. Guitarists used to use a Pignose (remember them?) to overdrive an amplifier. The Pignose uses germanium output transistors and input and output transformers, so it does not give the harsh sound and blunt clipping of typical transistor amplifiers.

It's also common to "mic" an overdriven amp for stage or recording studio sessions. Ray Davies of the Kinks used to mic a small practice amp and folklore has it that the speaker had a cracked cone.

Whatever the case, the amplifier and speakers become an integral sound of an electric guitar. There is a pretty big contrast between the rock and blues sound of an old school electric guitar performance, and the crunchy and compressed sound of electric guitars today. I prefer the former, but that's what I cut my teeth on. I do like a lot of contemporary music, but nothing beats the sound of a Stratocaster played loud through an old Marshall amplifier. Listen to Clapton on Derek and the Dominoes Live "Why does Love Got to Be So Sad"; Clapton manipulates the sound of his guitar from super clean to that bluesy wailing sound. I don't think you can do that with modern equipment.
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Old 10th September 2014, 03:53 PM   #4
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Derek and The Dominos - Why Does Love Got To Be So Sad (Live) - YouTube Wish I was there! But my parents wouldn't let me go to any live musical performances; that had to wait until college and by then Jimi Hendrix was dead. (How could anybody disrespect Hendrix?) My mother used to throw out my records when I was in High school; Clapton and Hendrix were demonic in her opinion. And don't get me started on my father.
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Old 10th September 2014, 08:28 PM   #5
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Thanks for your answer Fast Eddie D. I agree on the older rock sounds, they seem to be more organic than newer stuff although I really like e.g. the new sounds of Slash and the older stuff of Joe Bonamassa.

However I think we're talking about different things. For example Layla was recorded with a small amp (Fender champ if I'm not mistaken) and a lot of other guitarists use small amps in studio and on stage. But that's not what I'm aiming for. This is what I want: guitar -> big amp (50/100 Watt Marshall) -> dummy load/reamp -> speaker cabinet.

Provided schematics are for the dummy load/reamp part of the signal chain. I need a good buffer to reamp de knocked down signal, but due to lack of experience find it hard to design one. Suggestions and ideas are more than welcome.
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Old 10th September 2014, 09:17 PM   #6
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How about dummy load, attenuator, then a conventional amp?

Your dummy load is going to be big. I recommend an array of high wattage resistors.

I still think a small practice amp is the way to go. Marshalls sound best when they're stacked up on stage, with maybe 8 amps and speakers just for the guitar. That's how a lot of old school rock bands got their concert sound, before the days of outrageously huge amplifiers. It makes a signature sound too.
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Old 11th September 2014, 06:32 AM   #7
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by Rootz View Post
The reamping part is giving me a serious headache, as I'm just beginning with designing SS amps. It has to be a buffer (unity gain) with good damping and low distortion. This is what I've come up with and where I need suggestions.
Click the image to open in full size.
Your attempts are more or less workable, but if you need really low THD and high damping, you'll need feedback: a follower on its own won't be sufficient
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Old 11th September 2014, 10:19 AM   #8
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Originally Posted by Elvee View Post
Your attempts are more or less workable, but if you need really low THD and high damping, you'll need feedback: a follower on its own won't be sufficient
The dampng of those buffers will be enough for guitar speaker, the damping of tubes output stages is much lower and they sound beauty.

Better do not apply feedback in this case, the clipping will be much more pleasant
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Old 11th September 2014, 03:31 PM   #9
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What about the firts 3 schematics provided? Will they work and do you see room for improvement (stability, THD, damping). I'm not aiming for lots of damping or extremely low THD, but try to get the reamp more linear than the tube amp before the load. In theory that gives me a realistic load for the tube amp (loke a real 4x12" speaker cab) and a transparent reamp after the load. I was thinking that would be most like a tube amp running directly on a 4x12 (minus unlinearities because of voice coil heating etc.). Please feel free to correct me if I'm wrong.

I provide 2 LTspice files. In the first you see the buffer from the UA, followed by modded versions. The other file contains more ideas.
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File Type: asc Amp sims cfp vs ef 2.asc (17.4 KB, 8 views)
File Type: asc Amp sims cfp vs ef 5.asc (21.9 KB, 8 views)
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Old 11th September 2014, 07:21 PM   #10
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by Rootz View Post
What about the firts 3 schematics provided? Will they work and do you see room for improvement (stability, THD, damping).
They will work: as I said, they are basic, but workable, and you did your homework correctly.
The CFP version will unsurprisingly offer much better THD performance and damping than the two others, due to the inherent feedback, but if you under-dimension it in such a way that it will be allowed into saturation, it will be nasty.

You also have to account for practicalities: like adjusting the quiescent current. In sim, you just tweak the bias resistors, but in reality, you have to opt for something more practical than a dual trimpot f.e.
The Hfe of the transistors will generally not be nominal, like in sim, and you may want to reduce the bias and input resistors to take care of weaklings.
The DC offset is also something to keep in mind, in particular for the CFP version, where the matching of the bias resistors (and supplies) will be critical regarding that aspect.
A safe and easy option is to include a bipolar output blocking cap. Not very elegant, but effective.
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File Type: asc Rootz1.asc (17.4 KB, 17 views)
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Last edited by Elvee; 11th September 2014 at 07:25 PM.
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