Bass Guitar Amplifier Build - help and guide. - Page 1
Forum section: electronics / projects

author: Pexy13
author_rank: Regular Contributor
timestamp: April 04, 2019, 12:57:27 pm
content: Greetings.Since I recently started playing bass guitar I decidet to build my own amp insted of buying one.After searching the web for an appropriate schematic I found a page created by Dr. Bora Jagodić.(http://bas.elitesecurity.org/indexV.html)In it I found a schematic for a bass guitar amp and it seems perfect for my needs.For the power amp I decided to go with a TDA7294 as he sugested to use.I'll also build a 15V preamp PSU and a 22V power amp PSU.The schematic includes a ALC(automatic level controll) circuit that prevents clipping. It is optional but i would like to include it.But for more diversity can I include a switch that will connect and disconect the ALC circuit.I was thinking of inserting it before the drain of a 2SK30 fet and after the output of the power amp.Am I going to be able to achive what I intended or is there something different that I need to do to achive that?I will also insert a PDF with schematic and a Target 3001 file where I desinged the whole Amp.Thanks.Sven.

author: Zero999
author_rank: Super Contributor
timestamp: April 04, 2019, 02:06:24 pm
content: The symbol on your schematic is for a MOSFET, but the one on the PDF drawing is a J-FET.On your schematic, the drain is connected directly to the output of the op-amp which is wrong because the op-amp could go into current limiting mode, which would result in distortion. It should be connected to the other side of th 15k resistor, as per the PDF drawing.A switch in series with the drain would disable the AGC circuit, when it's open.

author: SiliconWizard
author_rank: Super Contributor
timestamp: April 04, 2019, 03:26:22 pm
content: Adding to the above, I'm personally not too keen on the original schematic's volume control arrangement.

author: 2N3055
author_rank: Super Contributor
timestamp: April 04, 2019, 05:16:07 pm
content: Quote from: SiliconWizard on April 04, 2019, 03:26:22 pmThat is actually quite common preamp gain control circuit. Master volume control would come before power amp.

author: Pexy13
author_rank: Regular Contributor
timestamp: April 04, 2019, 07:38:35 pm
content: Yeah, I conected the switch to the wrong side of the 15k resistor by accident.I will then use a single SPST switch in series with the drain.I don't know why in the pdf the symbol is for a J-FET when the 2SK30 is a MOSFET.Thanks.

author: 2N3055
author_rank: Super Contributor
timestamp: April 04, 2019, 08:09:01 pm
content: 2SK30 in questions N-Channel JFET and should be J-FET in that schematic. It is used as variable resistor.

author: Zero999
author_rank: Super Contributor
timestamp: April 04, 2019, 09:29:56 pm
content: Quote from: Pexy13 on April 04, 2019, 07:38:35 pmNo, it's clearly a J-FET. Look at the data sheet.https://datasheet.octopart.com/2SK30A-Toshiba-datasheet-101654.pdfIt will work with a MOSFET, but it needs to be a depletion mode device, i.e. on when VGS= 0V and off when VGS<< 0V.

author: Pexy13
author_rank: Regular Contributor
timestamp: April 04, 2019, 09:59:08 pm
content: Yes reading datasheet for the wrong thing and too quickly so I made a mistake.It's just so I can make a PCB layout and then the PCB itself.This is the only option I get in Target 3001 when I search for the 2SK30.Will it work for the PCB or do I need to change something?Thanks

author: 2N3055
author_rank: Super Contributor
timestamp: April 04, 2019, 10:26:29 pm
content: No that is not it ...Here it is..

author: Zero999
author_rank: Super Contributor
timestamp: April 05, 2019, 08:03:34 am
content: Quote from: Pexy13 on April 04, 2019, 09:59:08 pmThe datasheet I linked to is the correct one.It needs to be a J-FET. A MOSFET won't work, without changing the circuit.Yes, it's easy to find the incorrect parts, especially in this case as there are some MOSFETs who's part numbers begin with 2SK30, such as the 2SK3018, but they are not the correct parts to use in this circuit.

author: Pexy13
author_rank: Regular Contributor
timestamp: April 05, 2019, 12:40:28 pm
content: Alright. I will find one with correct leg placement to use in the PCB design and will be sure to use theright one in the project.You realy opened my eyes now for the stupid mistakes I made.Thanks.