j0nny_s

Hello,

I am currently building myself a power amp for my bass using a Velleman kit, http://www.velleman.be/ot/en/product/view/?id=360242 , and other bits and pieces.

What I would like to know is: if i remove the entire preamp section from my Laney guitar amp, would it still be compatible with a bass guitar?
Also, the preamp board would, I guess, need a few components replacing or removing (all I'm basing my idea on here is a project at http://sound.westhost.com/project27.htm#bass ).

A final question, could I run the preamp off the power transformer which is providing for the amp - without a significant power drain? Because (being a bass) I really need as much power to the amplifier as possible.. Transformer rating is [230vac in, 160vac out = 2x30v, 2x2.66A] part no. N04BA from Maplin

Thanks

Nigel Goodwin

I would suggest that amp is a bit under powered? - with only a single pair of TIP's I wouldn't advise using a lower speaker impedance than 8 ohms, so you're looking about 60W or 70W into 8 ohms. Bear in mind bass needs plenty of power! - if you're talking acoustic drums, and single 12 inch speakers, 120W for bass and 60W for guitar balances quite well.

For a simple (and popular) bass preamp I suggest you look at http://www.albertkreuzer.com/preamp.htm.

No problem powering it from the main supply, preamp power requirements are quite low, it's commonly done in that way - using crude zener stabilisers and wirewound resistors.

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Analog

I agree with Nigel, seems a bit underpowered, low headroom. You may have heat problems as well.

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j0nny_s

Yeah I know about it not being very powerful, but to be honest it was just gona be a temporary-ish amp for until I can afford something better.

As for heat, I'm going to order a 0.75c/w heatsink and mount a fan with it.

However, would it be possible to replace the power transistors with more powerful models?

audioguru

More powerful transistors won't change its max output power. The circuit has a current limiter that would need to be changed to a higher current limit. Then the more powerful output transistors would melt. Use a much bigger heatsink or a fan then the amp could drive more current into a speaker with a lower impedance for more power, or two speakers in parallel. You would also need to use a more powerful transformer.

If you want the amp to produce more power into the speaker you have, then increase its supply voltage and replace all the transistors to match the higher voltage. Probably most of the resistors would also need to be bigger so that they don't smoke. Bigger heatsink, bigger transformer again.

It ain't easy to increase the power output of an amplifier.

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Uncle $crooge

Nigel Goodwin

Normally not, but in this case it is quite simple, because it's specced into 8 ohms, adding a second pair of output transistors will increase it's capacity to feed 4 ohm speakers and (roughly) double the power. Obviously you require emitter balancing resistors, but it's a very simple procedure.

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audioguru

The Velleman amp is rated at 70W into 8 ohms or 100W into 4 ohms. Add a 2nd set of output transistors then it might provide 120W into 4 ohms. You won't notice the small increase in power.

Since it is already rated for a 4 ohm load, then using more powerful transistors and heatsinks or doubling them, using a more powerful transformer and increasing the trip point of the current limiters, then it will double its output power to 200W into 2 ohms. You probably cannot buy 2 ohm speakers so use two pairs of 4 ohm speakers in parallel.

You still might not notice the small increase in power. An expensive way to gain nothing much.

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Uncle $crooge

Nigel Goodwin

That's just it, it's NOT rated into 4 ohms, it's pushing a single pair of TIP's far too hard - to reliably feed 4 ohms you need a second pair of TIP's (check all the band gear which does exactly that) - such as Fender models, 60W 8 ohms with two TIP's, 100W 4 ohms with 4 TIP's (and a higher current mains transformer of course!).

Don't forget, bass guitar is pushing an amp HARD! - you need a good reliable design, pushing the outputs close to their limits doesn't give that!.

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audioguru

Yes, a guy on another forum recently smoked this Velleman amp with his 4 ohm speaker.
Did you notice that its distortion is rated for only 10W?

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Uncle $crooge

Nigel Goodwin

No, I can't say I had, but distortion should be pretty low until clipping anyway, it's a standard looking circuit - presumably 10W is just where it gives it's lowest distortion?

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audioguru

The Velleman amp is a simple circuit that uses resistors where a better circuit uses high impedance current sources. The guy who blew it up said that the LM3886 amp IC that he replaced it with sounds much better.

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Uncle $crooge

j0nny_s

Not meaning to bump or anything, but:

- would good cooling mean 4 ohm operation is safe for the amp?

- If so, I will use 100w 4 ohm bass speaker if the above is okay. Any suggestions to which model?

- is there that much benefit from using larger value supply capacitors. At the moment they are 3300uF. In the manual it is suggested 4700uF for "better performance". Is it worthwhile changing them, either to 4700 or higher?

Thanks for any suggestions

Nigel Goodwin

No, use four transistors rather than two as already suggested.

You'll struggle finding 4 ohm examples, most are 8 ohm - use two in parallel to give 4 ohms, and better bass as well.

Yes, particularly for bass!.

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j0nny_s

Okay, I think I will just leave it operating at 8 ohms in that case.