DIY Amplifier.

[davidturner]

Hi everyone. I have been having a go at building my own solid state amplifier. (Cant afford a new one!) Nothing spetacular, my own amp is only 10Watts! I bought an amplifier electronic kit (30Watts) and a pre-amp for it from Maplins. All the parts came to about £20. Trouble is when it is all connected up, the sound is barely audible. I think something is wrong with the pre-amp, mabey not letting the signal through. I have a bit of electronic knowledge but am struggeling to find the answer. I live in the Newtownards area of Co. Down. Can anyone help with the fault?
Thanks David.

[RogerCook]

Hi David

Which Maplin parts are you using? If you can post part numbers and details of how you have them wired up that would help

Roger

[Amanda]

Hi David,

As Roger says, if you can post info we can have a brainstorming session!

Amanda

[peter roest]

If the preamp is the K1803, the preamp is not suitable for guitar because the input sensivity is max. 40 mVolts. That to low for guitar. you always get distorsion.


Peter

[davidturner]

Thanks Guys, The parts I bought are as follows. The Amplifier is part no. N49FL. The preamp is part no. N47FL. And the speaker is part no. L72AW. All form Maplin Electronics.
I have a lab style power supply putting out 24vdc capable of 2.5amps. It is wired to the Gnd/Vs connection on the amplifier, and to the +/Gnd on the preamp. Next I have a mono jack socket connected to the preamp (Rin / Gnd). Output of that (Rout / Gnd) is connected to input of the amplifier (Vin / Gnd). Finally the speaker is connected to the output of the amplifier (Out / Gnd). I thought that being a stereo preamp, and using only one side of the preamp, may be causing problems. Therfore I 'bridged' out both left and right inputs,(so that the guitar is putting a signal in to both sides of the preamp) and the same with the outputs. Doing that I found it to be very slightly louder. But it is still barely audible. You have to put your ear about 2inch's from the speaker.
Thanks David.

[Amanda]

Hi David,

The TDA2050 is used in a few guitar amps including the fender frontman 15 and the
Vox Cambridge series.

It's probably a matter of drive, one channel of the preamp should work but if the sensitivity
of the amp is not high enough you will get little volume.

Also the input impedance of the preamp, if it is too low it will attenuate the guitar signal,
normal guitar impedance is around 250K to 500K depending on pickups.


Amanda

[RogerCook]

Hi David

As it's a stereo preamp you might try having the channels in series - ie connnect the guitar into say the left input, then connect the left output to the right input, then right output to power amp. I think I installed one of these preamps on my record turntable. I'll have a look and see if I can find the circuit diagram and see if that's sheds any more light

Roger

[Amanda]

Roger,

The PDF's are on the maplin site!

David,

Here's the circuit diagram for the Fender frontman 15 which uses a TDA 2050 in the
output stage, there's a bit more to it than the preamp you have built.

http://website.lineone.net/~m0dzo/Frontman_15G_Schematic_68F.pdf

It may give you some ideas.

You could leave out the "Drive Channel" components U2A and U2B and associated
components if you just want a "Clean" amp!


Amanda

[Didier]

The TDA2050 is used in a few guitar amps including the fender frontman 15 and the
Vox Cambridge series.

Yes, the TDA2050 is used in the Vox Cambridge 30, although some people replaced it by a LM3875 or a LM3876. Results are supposed to be very good (I have not checked).

Didier.

[davidturner]

How do Iknow if the input impedance is wrong in the preamp? What should I be measuring? Also If it is incorrect can any of the components be changed to correct it? If so which ones and what value?