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Old 8th September 2015, 07:34 PM   #1
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Join Date: Jan 2010
Default Unique project: guitar amp load, effects loop and reverb all-in-one

WARNING: long story, but certainly worth reading, especially for guitar players!

Let me first shortly introduce myself: I'm Bas Kooij, Dutch, guitarist and a long time electronics and classis rock lover. I almost exclusively use vintage (or cloned vintage) guitar amps: non-master volume Marshalls, 2204 JCM800's, Ac30's. I always loved that rich and organic sound they produce when turned up really loud.

Those amps have their shortcomings nonetheless. They are essentially extremely loud distortion boxes the way I (and countless others) use them. To get the full potential you have to turn up the volume and overdrive the phase inverter and power tubes. I recon overdriving is an absolute no go in Hifi, but not so much in guitar amps. Yes, there are guitar amps which get their distorted tones only from the preamp, e.g. the Soldano SLO or the Bogner ‹berschall. Both great sounding amps, but not what I'm looking for. I want my vintage stuff with some added flexibility.

Now there are some product on the market to load a guitar amp and re-amp or pass through a small portion of the signal. They are called attenuators and they have flooded the market in the past years. I've used the Ultimate attenuator and the THD Hotplate. Both are doing what they should: bringing the volume down to manageable level even when the preceding guitar amp is putting out 50 or more Watts. But both also alter the sound tremendously. The Hotplate tends to (to my ears!) sounds tinny when use on any setting below -8 dB (which is still extremely loud!). And the Ultimate cuts large amounts of high and low frequencies. And there are more shortcomings. Both only offer a line out for e.g. re-amping, but no other features to shape the tone or introduce effects like delay or reverb.

However the Ultimate attenuator is of a pretty clever but simple design. It uses a 30 Ohm load and the signal from the load goes into a potmeter to a simple emitter follower buffer amp (2SA1943/2SC5200 pair). The load is pretty high and not as realistic as a loudspeaker would be, but it works. I took the design one step further and redesigned the load to be electrically the same as the speaker cab I use, including the resonance peak and climbing impedance towards higher frequencies. That way the amp essentially sees a speaker as a load and with the low output impedances of the guitar amps involved that's a huge difference. There was the missing bass and treble of the original Ultimate attenuator!

And now again I want to take the design a step further. What's missing is a built in effects loop and reverb. Especially vintage Marshall (which I love most) miss this feature. It is possible to build an effects loop in the amp. Metro amps and other suppliers have got solutions for that which are using high voltage MOSFETS. IMHO there are two problems with that approach: 1) the loop is introduced before the phase inverter and power tubes which means parts of the amp after the loop still contribute to the overall distortion of the amp. This makes it less than ideal for time based effects like delay and reverb where you want all the overdrive in front of those effects. Things can get easily out of hand when reverb and delay are in front of distortion effects in a way that it sounds unmusical and messy. 2) drilling holes in vintage gear is not a good idea IMHO. Would you drill an extra hole in a otherwise mint 1961 Stratocaster? I recon that most amps don't are as valuable as e.g. an original 1961 Strat or a 1959 Les Paul, but you get the point.

Now what if I could implement a effects loop and reverb in the attenuator? Bryan Ho did that with at least one of the Ultimate attenuators (the effects loop I mean), but in a more unconventional dry/wet setup. The attached picture shows my take on the implementation of the new features. There isn't anything groundbreaking to most parts of the design, but I definitely need some help to check the design. Apart from what I read the past month I've got no experience in designing with opamps.

The design goals are:
1) Keeping costs and required parts down;
2) Pretty much bullet proof;
3) Working from a single PSU, so probably only discrete and/or opamps;
4) Loop must be able to do line levels between instrument level and pro audio level (100 mV to 1.4 V);
5) Loop must be bypassible;
6) Reverb must be based on 310 input en 1575 Ohm output impedance tank (accutronics 8DB2C1D);
7) Able to drive a wide range of load from 4 to 16 Ohms (the LM3886 seems suboptimal for this purpose).

As I said, I'm a novice when it comes to designing circuits with opamps. For starters I'd like to know if there are any obvious failures in the schematic and if there are certain obvious improvements possible.
Attached Images
File Type: png Screen Shot 2015-09-08 at 20.32.47.png (473.3 KB, 29 views)
Attached Files
File Type: asc opamp buffer v2.asc (42.3 KB, 3 views)
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