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 1 
 on: April 27, 2017, 07:58:53 AM 
Started by galaxiex - Last post by teemuk
For example:


As you see, it's pretty much the generic current feedback topology. In just few years this basic circuit would convert from being a short circuit protection to being a tube amp emulation feature instead. Simultanously gaining worldwide popularity as that.

Yes, information was much harder to acquire back then. Therefore I beleive there must have happened something very groundbreaking that explains why this scheme spread to numerous solid-state guitar amps like a wildfire in the early 1970's - and particularly NOT as a protection feature but as a tube amp emulation feature.


 2 
 on: April 27, 2017, 05:42:47 AM 
Started by baaron31 - Last post by J M Fahey
Have you checked you have NO DC at that jack?

Where/what/how is it connected to?

 3 
 on: April 27, 2017, 05:40:19 AM 
Started by galaxiex - Last post by J M Fahey
Quote
However, the circuit documentation still describes the associated circuit as a crude short circuit protection scheme.
Well, thatīs what it originally was.
And not *that*  crude, in its original implementation it was functionally the same as the standard current clamping protection with diodes or transistors.
Way back then, we knew it as "the RCA protection" .
FWIW, found in transformer driven SS amps.
The original idea was never to increase output impedance; amp was standard high damping type, as expected in a Hi Fi amplifier, and started limiting current only above a certain preset (dangerous) level.
Will search for some old schematics ... printed on paper of course ;)  , no Internet way back then :O

 4 
 on: April 26, 2017, 04:24:53 PM 
Started by baaron31 - Last post by baaron31
Ok. Wasn't aware of that! I tried it in two different passive cabs and hummed. Then i tried in into the input of an old Yamaha G100 and no sound came out. I will try it into another powered amp and see what happens. Thanks again G1, your help is appreciated.

 5 
 on: April 26, 2017, 04:21:18 PM 
Started by baaron31 - Last post by g1
Signal out is to go to another power amp or a powered speaker.  Is it a powered speaker you are running the 'signal out' into?

 6 
 on: April 26, 2017, 12:56:53 PM 
Started by galaxiex - Last post by teemuk
When I started to research all this, what I discovered was that many of these kinds of features (current feedback) were actually introduced much earlier than people commonly think.

There's nothing new in mixing in different feedback topologies. A lot of stuff about that was discovered already in the early days of tube amps and some of the inventions are still exploited. Certain Bogen tube amps from few decades past were famous for employing -positive- current feedback because it - like negative voltage feedback - decreases effective output impedance of the amplifier. If interested, you likely find a few related magazine articles and patent documents with a Google search. Anyway, what the solid-state stuff is doing is just inversion of the idea: Apply negative current feedback, because it increases effective output impedance of the amp.

Overall this scheme must have not been -that- uncommon for amplifier designers. Especially for those who earn a living with it.

Anyway, I wonder what groundbreaking happened in the late 1960's, because after that we see a boom of this basic "current feeback" -topology making an appearance in many solid-state amps, and yeah, particularly as a deliberate attempt to emulate tube amps.

Ca. 1969 Triumph introduced amps that employed negative current feedback and had a moderately high output impedance. However, the circuit documentation still describes the associated circuit as a crude short circuit protection scheme.
Then all suddenly... The self-powered cabs of the Fender Super Showman system (ca. 1969) employed current feedback and associated tube/SS -tone switch. Jordan amplifiers (early 1970's) employed current feedback, with or without adjustment. Randall, Risson, Polytone, etc. In early 1970's the scheme is employed all over the place, and I even heard that there is a German book about solid-state guitar amplifiers from the time that describes the scheme. Several early 1980's patents (at least european ones) refer to the scheme as well-known "prior art" so basically every designer for every bigger brand out there must have at least been aware of this stuff, whether they chose to implement it or not. I wonder what happened?

Let's put this to proper context in time line: All this stuff happens about 15 years -before- the famous "Carver challenge".

 7 
 on: April 26, 2017, 08:40:13 AM 
Started by bancika - Last post by DrGonz78
 :dbtu: Yup your program makes it so much easier to translate a schematic into a layout when designing an amp. Thank you so much for this great software!!

 8 
 on: April 26, 2017, 12:59:30 AM 
Started by aoresteen - Last post by J M Fahey
Cool  :dbtu:

In a nutshell: I think the amp "as original"  is just on the edge, too close for comfort, to drive 4 ohm loads.

Not that it canīt absolutely drive them, otherwise they would not have released them, but real world touring or even plain stage use (LOUD, matching a LOUD drummer) are hard on an amp.

I guess that you can add 2 more extra devices or replace the current ones with double chip Exicons, which work very well.

Whenever possible show us whatīs making it tick :)

 9 
 on: April 26, 2017, 12:41:42 AM 
Started by bancika - Last post by J M Fahey
Thanks A LOT for all your useful efforts to help DIYers   :dbtu: :dbtu: :dbtu:

 10 
 on: April 26, 2017, 12:39:43 AM 
Started by galaxiex - Last post by J M Fahey
Thanks.
Looks like Jordan was quite the pioneer, this amp has the basic structure of a Polytone amplifier.
Amazing.

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