Hey everyone, I've been working on an 1x12" amp design and learning a LOT in the process. My "reference" amp is a Fender Hotrod Deluxe which I do like. I'm not trying to emulate it with solid state circuits, but it has been a good reference point so far.

One thing that's baffling me is the "presence" control. On the Hotrod I always have it cranked... I find things sound terrible otherwise. I've plotted the curve of the Fender power amp with the presence control cranked and have been trying to get something similar in my power amp. I know that the presence is part of the current feedback circuit, reducing the feedback at the higher frequencies. I'm having trouble though when it comes to knowing how much feedback I should be using in my own circuit.

With respect to current-mode feedback, I have implemented it around my amp using a 0.25R resistance in series with the 8ohm speaker. I have buffers and such set up to amplify the recovered signal from the shunt and mix it back into the signal feeding my amp. I use a dummy load that is switchable to 8, 12 or 24 ohms and I've been using this to see how the load affects the output signal from the amp. When I change from 8 to 24 ohms the amp output changes by about 3-4dB based on my configuration. But I have no idea what is the "normal" amount of feedback to apply, and how much my load should affect the output voltage of the amplifier.

Can anyone explain what is the normal amount of feedback and how much it should affect the output of the circuit? So far I haven't come across any hard and fast numbers based on practical implementations. Thanks!

Hey all,

I'm pretty experienced with the classic filter types and putting together various active and passive filters for simple things. But here's something I've recently run into that I haven't experienced before: converting a known (desired) complex EQ curve into the simplest circuit possible.

Following the advice of something I read, I've been using a 31 band graphic EQ to shape the sound going into the power amp section of my project. I've also found a great windows program called Tombstone for measuring and printing response curve of real circuits. But I'm having trouble figuring out a systematic (automated, programmatic?) way of producing a circuit that has roughly the response of my desired curve. Do I make a gyrator for every peak and valley and tweak each one like in a graphic EQ? Or should I make multiple stages, one for each boost/cut? Simple rolloffs are easy, but I need a few bumps and dips in the response here and there.

What method to people use for this sort of thing? Any insight would be greatly appreciated.

I'm trying to keep my SS amp design super efficient (I want it to run on batteries and be super light) so I've been checking out Class D amp ICs. I know they've gotten a bad reputation in the past for being high distortion, but it appears that new ones are getting pretty good. Obviously you want to keep it out of clipping and the PCB layout and output filtering is a bit more of a challenge, but the power supply and heat savings are very impressive.

So, I've been checking out the TPA3106D1 from TI. Looks like a great part, and works with a single supply. Can do >30W into 8 ohms with <1% THD from a single +24V supply at >90% efficiency.

How much power do most people feel they need for a solid state power amp? Mine will be driving a 1x12" open back combo with a speaker rated at >100dB SPL 1W/1m.

Does anyone have other recommendations of parts or have experimented with Class D at all?

I'm very happy to have stumbled upon this forum. Having grown up in the 80s and learned about electronics long after tubes were in museums, it's great to find others who are interested in solid state amps! I've been working in the electronics business for a few years now mostly doing designs of non-music stuff. Due to sheer boredom and the desire to make musical instruments I've started working on some designs for guitar electronics. My main project is a highly portable 1x12" combo amp with a number of interesting features. I hope to make this into a "boutique" product as a side business to support my hobby.

Strangely enough as much as I love modern electronics, I got roped into the tube craze when I bought my current guitar amplifier. (a Fender Hotrod Deluxe) It really isn't all the great as a practical amplifier. It's too loud for practising, (which I've mostly solved with an attenuator in the effect loop... the speaker attenuator I built doesn't seem to sound as good) doesn't have a headphone jack, has a spring reverb (which I hate the sound of) and generally has a very small range of usable tones. (not good for an eclectic musician) Also, I can't ignore the fact that it weighs 45 lbs and is terribly inefficient.

So when starting to design my amp I pretty much ruled out a tube power amp due to weight and size. And then that left the choice of a tube vs. solid state preamp. I built a little tube test system for quickly (and safely) setting up circuits with a pair of 12AX7s. My ear tells me after a lot of experiments that I don't necessarily like the tube sound better. I've got some fairly good sounding preamp circuits working with opamps, and a nice small hardwood ply open-back box with an Eminence Lil' Texas (neodymium magnet) speaker inside.

My question for you all is this:

Can a solid state amp work as a boutique, high-end product? Would you pay $1000-1500 for a low-run, great solid state combo amp that has exceptional sound and features?

Since I've designed a lot of commercial stuff, I can't think of a hobby project as a one-off anymore. Instead I want to build stuff that other people will want to purchase and enjoy. It all seems more worthwhile that way.

All comments are greatly appreciated!