Bass guitar amp design guidelines

[Bill622]

Hi,
I would like to know if anyone could suggest me a schematic for a bass guitar amp. I'm looking for a Class-D type of amp of around 100W, or something simple without tubes. I have found this design but I don't know if it's suitable for a bass guitar:

http://www.hobby-hour.com/electronic...-amplifier.php

If I take my question in an other way: what is needed for a bass guitar amp? I'm looking for a wide frequency response, I don't want EQ's at this stage of the project, just an amp is a starting point...

Thanks!

[StefanTH]

Originally posted by Bill622
Hi,
I would like to know if anyone could suggest me a schematic for a bass guitar amp. I'm looking for a Class-D type of amp of around 100W, or something simple without tubes. I have found this design but I don't know if it's suitable for a bass guitar:

http://www.hobby-hour.com/electronic...-amplifier.php

If I take my question in an other way: what is needed for a bass guitar amp? I'm looking for a wide frequency response, I don't want EQ's at this stage of the project, just an amp is a starting point...

Thanks!

Apart from a high power rating (100W or even 170W is a bit on the low side for anything but a practice amp to use at home in my opinion), there's nothing special about power amps for bass guitar. Indeed, some players even use power amps intended for PA use in their more ambitious rigs. In other words, if the design you posted is suitable for ordinary sound it's suitable for a bass guitar. Whether 0.3% THD at 10W (for a 170W amp) is suitable for sound or not I'll leave up to you.

[Bill622]

Thanks for your answer, but I would like to understand something. When I plug my bass (passive pickup) into a mixing console it sounds really bad. I use a gain controled input and it seams like the amplitude is irregular, the attack of the note is really strong and then level seams to drop very quickly. I assumed bass amp were not linear but logarithm, at least for the preamp. I might not use the right word, for me, logarithm amplification, the gain is higher for low level input sound and the gain become lower for high level input sound.

Am I wrong?

[sreten]

Hi,

Your talking compression and limiting, both useful for bass amplifiers.

Probably you can get way with using a decent bass pedal as a
preamplifier into that class D amplifier, and setting up the pedal
to prevent overdriving (by limiting) and increase sustain (by
compression).

The pre-amplifier does need all the features, though in my bass
amplifier it seems limiting is built around the power amplifier.
(Peavey mk111)

/sreten.

[StefanTH]

While some players like to have a compressor in their signal chain, few want one in their amp. On-board compressors are usually rather poor (cheap, lack of control, doesn't sound very well) and what's more, you're stuck with it at the end of the signal chain so using other effects (and making them sound decent) becomes a real pain. A gain stage (max gain +40dB seems to be the most common) and an EQ is enough for the preamp section of a bass amp, though a post-EQ FX loop is not uncommon.

On what input of what mixing console did you plug in your bass? When connecting a bass to a mixing console, you should use a D.I box (an active one in my opinion). It's not supposed to sound bad of course, virtually all live sound engineers go bass-D.I-mixing console, but the other part (strong attack, fast decay) sounds like a pretty accurate description of bass guitar with agressive playing to me.

[Bill622]

Thank you all for your answers, you've all brought very interesting points.

My mixing console is a cheap Behringer Eurorack UB802. I used one microphone input (1/4" jack): gain, eq and level control. But you're right, I did not used any direct box. It might be the source of poor sound.

After I posted my message about logarithm amplification, I continued to read and I've found out that compressors are like that. I tried an external compressor once, (rack mount). The control available on that were amazing, i'll never consider a basic, integreted compressor for a bass amp, for me, external is more flexible.

If i get back on the main idea of this thread, here is my comprehension of what I should do:

Step 1: making a good amp with less noise as possible. Is a Class-D any good for a bass amp? I like the design I've found (link in my first post) but I was looking at National's website and these parts are now discontinued without any direct replacement. Should I consider the same king of amp with a class-d amp combined with power mosfet or standard transistor would be better? I'm not convinced that a LM3886 based amp would be good.

Step 2: making a pre-amp where i would have EQs (at least 3 band)

Step 3: making a 2 or 3 way cab with subwoofer/mid/treble. I just don't accept single 15" driver cab, a bass guitar still produce a wide frequency range.

Any suggestion about the Amp section?

Thanks!

[StefanTH]

Class D can be good for a bass amp depending on what you're after. The advantages, as you're probably aware, is high efficiency which leads to a potentially lighter amp and heat sinks being less of an issue (though still a minor issue). What you'll have to live with in order to reap those rewards is a higher THD (just a guess this, but spontaneously I'd say 0.3% THD from a Class D sounds a bit harsher than 0.3% from a Class B too, anyone with more knowledge than me care to comment?) and a more complicated design that will trouble you with all sorts of unusual stability issues before you get it right.

Class G and Class H are more popular in bass amplifiers than Class D, both being somewhere between a Class B and Class D in terms of efficiency as well as THD with Class H closer to a Class D in those respects.

As for #3, there are a number of interesting opinions on cabs for bass guitar. Everything from Phil Jones' 32x5" to some 3-way designs using 21", 5" and 1" elements and literally hoards of pseudo-science defending each way to go. For 1x15" users, it's not so much about what frequency range is produced by the bass as how the 15" element shapes the sound (in other words how it cuts most of the HF content etc) in a way they like. Also, full range cabs don't lend themselves all that well to some styles of music, just as they sound better in other styles (provided the player's technique is up to par).

[Bill622]

Originally posted by StefanTH
Class D can be good for a bass amp depending on what you're after. The advantages, as you're probably aware, is high efficiency which leads to a potentially lighter amp and heat sinks being less of an issue (though still a minor issue). What you'll have to live with in order to reap those rewards is a higher THD (just a guess this, but spontaneously I'd say 0.3% THD from a Class D sounds a bit harsher than 0.3% from a Class B too, anyone with more knowledge than me care to comment?) and a more complicated design that will trouble you with all sorts of unusual stability issues before you get it right.

Class G and Class H are more popular in bass amplifiers than Class D, both being somewhere between a Class B and Class D in terms of efficiency as well as THD with Class H closer to a Class D in those respects.

As for #3, there are a number of interesting opinions on cabs for bass guitar. Everything from Phil Jones' 32x5" to some 3-way designs using 21", 5" and 1" elements and literally hoards of pseudo-science defending each way to go. For 1x15" users, it's not so much about what frequency range is produced by the bass as how the 15" element shapes the sound (in other words how it cuts most of the HF content etc) in a way they like. Also, full range cabs don't lend themselves all that well to some styles of music, just as they sound better in other styles (provided the player's technique is up to par).

There is indeed a few designs between a single 15" and a 32x5". I think it's a matter of personnal opinion. I like a clear sound, I never use the tone control on my bass. I also am a luthier and each bass I made sounds different but there is a big difference between my amp and the "acoustic" of the instrument. I would like to be able to reproduce the real voice of the instrument. This is why I think a cab should be made of different range drivers.

I don't know much about Class-G or H amp. I made a few research and I have found this: Philips TDA1562 Class-H. It is only 70W, I was looking at something with a bit more punch. So I've found this: Philips TDA8950. I know it's Class-D and it would need a nice pre-amp but it sure can push enough power.

Thanks for your answers, whenever i start building an amp, i'll update this project.

[PeteMcK]

"When I plug my bass (passive pickup) into a mixing console it sounds really bad" - you need a high impedance input or a High impedance buffer, mic inputs are low impedance

[Nigel Goodwin]

Originally posted by PeteMcK
"When I plug my bass (passive pickup) into a mixing console it sounds really bad" - you need a high impedance input or a High impedance buffer, mic inputs are low impedance

Yes, a mixer input is completely wrong - but all you need is an Active DI box - this gives an un-balanced high impedance input, and a balanced low-impedance output.

I use a Behringer DI-I20 two channel DI box, and put both guitar and bass through it direct to the mixer (for small pub gigs as a duo). It's a handy tool to have in your gig bag, in fact I bought a second one it was so useful but never needed two yet