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 1 
 on: February 20, 2018, 08:30:30 AM 
Started by dazz - Last post by dazz
Thank you for the detailed explanation and your patience too, Phil  :cheesy:

 2 
 on: February 20, 2018, 07:40:03 AM 
Started by dazz - Last post by phatt
As long as R17 and R18 are the same value it will still give you half voltage at the bias node. R2, R7, and R12 give each input a ground reference to work from so the signal can swing evenly pos and neg of the bias point.
(opamp basics, the positive input always needs a DC path to reference, in this case it's the half voltage bias node)

R7 can be a bit higher or lower than 10k,, lower values may pull down the gain, too high and it may effect the filter network in front of it.
R12 needs to be as big as possible, lower values there will effect how well the tone controls work. Try 2meg if you want.

R14 along with R13 effects the gain of U3, higher values at R14 will lower the gain while higher values of R13 will increase the gain.
R15 and C16 pull a bit more treble from that stage.

If you use a bread board to test you can tweak the values till you find the best outcome for you needs. 8|
Phil.

 3 
 on: February 20, 2018, 06:04:11 AM 
Started by dazz - Last post by dazz
I'm afraid I have another question (hopefully this one is not as dumb as the previous ones). I don't have many 10K resistors right now so I'll be using something higher for the bias resistors. I understand R7, R17 and R18 are bias resistors, but what about R14 in the 3rd opamp? There was a 10K resistor there in the original schematics so I'm not sure if that needs to be matched to the value of R7, R17 and R18

 4 
 on: February 20, 2018, 03:58:18 AM 
Started by dazz - Last post by dazz
Yeah, I should have figured those out myself, sorry about the dumb questions.
I knew in the back of my mind that 3rd lug in the DC socket was there for a reason *blush*

 5 
 on: February 19, 2018, 11:03:46 PM 
Started by dazz - Last post by phatt


I don't think I want anything over 15V through the TDA7297 since that's when it reaches 10W and it starts to distort (according to the datasheet),

whatever the max supply voltage reads on the data sheet,, don't go above that.

but let's say I add a DC input jack so that I can run the amp off the internal 12V psu or this external supply: that would put 15V (or more) at the output of the disconnected internal 12V supply. Can this damage the 12V supply?

Sorry but without a drawing there is little to go on.
If the internal suppy is *Disconnected* then I assume there is no problem powering from an external supply.
Most pedals have a switch built into the DC socket so when you insert an external wall wart the battery is disconnected.

Also, what's the maximum Vdd I can feed your Casino 12 please?

Whatever the data sheet reads for the chips you are using.
Most will run on 30VDC or 15-0-15VDC.
Phil.

 6 
 on: February 19, 2018, 08:57:22 AM 
Started by dazz - Last post by dazz
Maybe add a 10~47uF cap on the supply.
With such a small case you have to watch out for inductive coupling which can cause all sorts of problems.
Also if the battery is old that might cause problems.
Better minds here may know more.
Phil.

Thanks Phil. I already have a 100uF cap at the supply, per TDA7267A schematics. Pretty sure the battery is at fault, it's fresh but a cheap one. Measures 9.5V unplugged, goes down to 8.8V as soon as I connect it to the amp, and down to 7.7V when I strum a chord at full volume.

Well, sorry to be such a pest but I have yet another question about the larger amp (Casino 12 + TDA7297). I bought me a power supply, one of those universal laptop chargers that runs at anything between 12 and 24V which seems to work really well and dead silent after grounding the DC negative output.
I don't think I want anything over 15V through the TDA7297 since that's when it reaches 10W and it starts to distort (according to the datasheet), but let's say I add a DC input jack so that I can run the amp off the internal 12V psu or this external supply: that would put 15V (or more) at the output of the disconnected internal 12V supply. Can this damage the 12V supply? Also, what's the maximum Vdd I can feed your Casino 12 please?

 7 
 on: February 19, 2018, 08:19:11 AM 
Started by dazz - Last post by phatt
Maybe add a 10~47uF cap on the supply.
With such a small case you have to watch out for inductive coupling which can cause all sorts of problems.
Also if the battery is old that might cause problems.
Better minds here may know more.
Phil.

 8 
 on: February 19, 2018, 06:47:12 AM 
Started by dogbox - Last post by dogbox
Thanks heaps Phatt. Had it sideways...yep soldered on 6 wires the wrong way. Switches are my Kryptonite  :duh :loco

 9 
 on: February 19, 2018, 06:27:23 AM 
Started by dogbox - Last post by phatt
If you have a meter then check which terminals are contacting in the 2 positions.
This edited circuit might help;
Phil.

 10 
 on: February 19, 2018, 06:23:04 AM 
Started by dogbox - Last post by Enzo
The 1 meg resistor?  It is one million ohms in series with the signal to knock it down to almost nothing.  Note the 1 meg is only in the circuit during clean.  Clean is when the OTHER path is used.  When you want overdrive channel, the switch shorts across the 1 meg, thus removing it from the circuit.

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