Amp design - opinions please

[Nick Ellingworth]

I've been keeping this under my hat until I had most of the design pinned down but as part of my second year at uni I am undertaking a half year project in preparation for the dissertation/1 year project in year 3. For this project I am designing a portable guitar amplifier, the requirements of this are that it must be battery powered and have it's own speaker.

So my basic plan initially was input -> pre amp -> power amp -> speaker. Nice and simple.

However during the pre amp design stage I have started investigation the potential of the EQ section, primarily I find my self wondering why so many amps are limited to a singe EQ style (eg: the usual Fender or Marshall sounding EQ), "why not have 2 EQs and allow the user to switch between them?" this in turn led to "hey blending would allow for in between tones."

So I've now got:

                        EQ1 (Fender)
Input -> Blend ->                      -> Pre Amp -> Power Amp -> Speaker
                        EQ2 (Marshall)

A nice complex project. However I would like some feed back about the EQ idea before I commit to it so what are your thoughts?

 

[dbw]

Sounds fine, but traditionally the EQ comes after, or is part of, the preamp.

Not sure how useful the blendiness would be.  I think I would just have 2 preamps on 2 channels, one with Fender EQ and one with Marshall EQ.  Then you could stomp between the preamps to switch EQ, and also set different gain levels on them to stomp on distortion.

 

[mayfly]

In my designs, the most useful feature was switching between different gain/volume settings in the preamp.  The result of this was that different gain/overdrive settings required different EQ, hence adding a different tone stack in each "channel".

BTW, there is really not much difference between a Marshall and a Fender tone stack.  If you want different, have a look at HiWatt.

 

[Nick Ellingworth]

Actually there is quite a large difference is the way the EQs respond to different frequencies, I've been looking at spectrum analysis of both and there is enough of a difference to justify both (even if it's not easy to hear) from a technical point of view. Although from a construction perspective they are nearly identical as they are simply modified Baxandall circuits.

I have no intention of adding distortion to this amp it's going to be completely clean.

Thanks for the thoughts they're a lot of help.

One disappointing feature of this project is that due to the battery power limitation I'm only going to be getting a measly 0.1 or 0.2 watts of audio power.  :-\

 

[mayfly]

I'm assuming this is a solid state design?

Don't forget that you can do a lot with a 99db/watt speaker...

 

[Nick Ellingworth]

Yeah it is solid state, I don't have the time to muck around with transistors.

I can't get a speaker quite that good but 85-90 db/watt is possible. Thanks for the reassurance there as I was worried that I might end up with an amp that's barely audible. Still I have discovered that I might be able to get 1 watt of audio power out of the lm386n4 chip if I use 18v (2 9v PP3 batteries) with out affecting battery life too much. So a potential of 85-90 db from a tiny little amp is a rather nice thought.

 

[dbw]

A 9V battery powering an LM386 can drive a 4x12 to about talking volume.  (You'll have poor battery life at that volume, though.)

 

[mayfly]

Yeah it is solid state, I don't have the time to muck around with transistors.

I hate to tell you this son, but transistors ARE solid state.  Good thing you're still in school - I'd have to smack you one if you had said that after graduation.

 

[jimh]

Yeah it is solid state, I don't have the time to muck around with transistors.

I hate to tell you this son, but transistors ARE solid state.   Good thing you're still in school - I'd have to smack you one if you had said that after graduation.

+1  Whoops :doh:
:icon_tongue:

 

[callaway]

Just do a simple push-pull with MOSFETs. You'll be able to get a lot more power than with the LM386. Of course, you need a lot bigger battery as well...

Do you have a target weight? I'd use some different batteries, as you're just not going to get much life using 9 V batteries. Maybe you could even try a motorcycle battery if you want some real lifetime and power.

Are you intending to achieve any sort of drive or distortion? I might consider taking the clipping stage from a tube screamer and making it a part of the drive channel preamp.

 

[Nick Ellingworth]

Yeah it is solid state, I don't have the time to muck around with transistors.

I hate to tell you this son, but transistors ARE solid state.   Good thing you're still in school - I'd have to smack you one if you had said that after graduation.

+1  Whoops :doh:
:icon_tongue:

I do know that transistors are solid state, I was tired and not thinking straight when I wrote that post.

 

[Wana_make_a_guitar]

Well why do you have to use 9 volt batteries? Can't you use D size batteries instead, it's not like it'll be that heavy.

 

[Nick Ellingworth]

I'd need 12 of them to power this amp.

 

[Wana_make_a_guitar]

I'd need 12 of them to power this amp.

Oh  :confused4:

 

[blue313]

Maybe you could even try a motorcycle battery if you want some real lifetime and power.

This is true, what are the limitations of his legal definition of battery?  A cordless drill battery is another thought, they have nice 18v battery packs.


This sounds like a smokey amp.

 

[Wana_make_a_guitar]

Ah, I was gonna buy a smokey amp, they double as a fuzz pedal!

 

[Nick Ellingworth]

This sounds like a smokey amp.

My amp design is a lot more complex than that.

 

[blue313]

My amp design is a lot more complex than that.

Lol true, I meant to quote dbw's earlier post on the lm386.   Ive seen a 2x12 pushed with one of these and it wasnt bad....for a few minutes.

 

[mayfly]

The trick is to not let the smoke out.

Actually, if you want low power consumption, stay away from op/amps etc.  The lowest power circuits around are the ones that you have control over.  Simple common emitter circuits for the pre-amp and a totem pole mosfet output stage will likely yield the best battey life.

BTW, are you using all those batteries to get a two rail supply?  If so, with discrete circuits you won't need a negative rail if you do it right.

 

[dbw]

Use a truck battery... you'll get like 100 amphours at 12V