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Topics - dogbox

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Preamps and Effects / Old newb questions
« on: December 20, 2020, 04:42:04 PM »
Hey guys, haven’t been here in a while or played around with electronics and electric guitars for couple years. Been on an acoustic odyssey lol. Feel like I’m starting from scratch again. Dragged out my MIM strat and red knob superchamp..thats downstairs in the man cave and too loud for the living room.

I was wanting to build myself a small stereo coffee table guitar amp. I have seen plenty of cheap amp boards on ebay, pre soldered with surface mount components with stereo 3w output. That would do me, anything much louder would annoy the missus, and we don’t want that ;) and I don’t want make a real amp with power transformer..i don’t do AC.

I need a preamp...but before I get to that stage I have some really obvious things that I don’t know/ forgotten.

1. Could I just use an effects pedal as a pre?
My intuition says no because guitar amps have an input stage ( i remember that part)

2. Would pedals work directly into a line level inout if i placed a DI box between pedal and amp? What would it sound like? Would a cabsim pedal or amp in a box work?

3. Is there a schematic on here or elsewhere that gives a simple clean channel, with maybe basic tone controls that you could point me to, that may be suitable for these ebay amps. Fetzer valve was one i used in the past and I remember Phil(Phat) had some great preamp designs floating around here. Would there be a way to modify one of those to work with the ebay amp?

4. Should I stay away from those chinese prepopulated boards on ebay, and build a proper deigned practice amp? If the latter option, Which one?

5. If it was Ok to have a preamp board ( that i would perf) and the ebay amp side by side in a box, and i didn’t need a D/I circuit in between, could I use a single 9/12v wall wart to power both boards, or would I need a seperate supply for both pre and power amp?

Am i barking up the wrong tree with the D/I box..i just remember having a need for a DI box when plugging into my mixer years ago..sounded terrible without it, and it had line level inputs. When it comes to electronics I know enough to be dangerous. I can solder ok and use a dmm. I used to build pedals 10 years ago, then had sporadic forays back into electronics but only limited understanding. Sorry for the dumb questions, and rambling in advance

Looking for your input

Amplifier Discussion / Marshall MG10 Switch
« on: February 18, 2018, 10:25:30 PM »
Hi Guys,
I bread boarded the first 2 stages of this and individually each channel sounds good. I changed it to suit a single 12v supply. Sounds Ok anywhere from 8v to 12v..with my AN7523 chip power amp. Eventually will make a good portable practice amp.

Just scratching my head about the switch found at the start of the gain stage.  I'm using a 3 leg, single pole double throw toggle. I put the centre leg at the output of the first stage, then the both outside legs just before the 10 uf decoupling caps of both clean and gain paths. The switch works, but I get some cross talk between the 2 channels, for instance turning the pot on the clean channel changes the the volume slightly on the gain channel, but in reverse. i have to reduce the clean channel to get the most volume from the know what i mean..the signals aren't separated properly. Also I don't understand why in the schematic, the switch straddles the R9 1M resistor?? Before I put the switch in the 1M completely changed the the gain stage to "clean" nice fuzzy distortion. I left that out, but now thinking I need that in there.

I just can't get my brain around where to place the switch leg.
Here's the problem, If i put the 1M in the circuit as shown, and place the the switch after the 1M, i lose de-coupling from that stage. If I put the switch after the de-coupling cap, but before the 1M i lose distortion??? Can someone explain what I'm doing wrong here?? Am I using the wrong type of switch?? I have a bunch of the SPDT switches and a few 3PDT (effects pedal stomp switches)..would a DPDT switch be the correct type???

Amplifier Discussion / Opamp slew rates
« on: February 06, 2018, 04:10:04 AM »
Hi guys,
I'm wanting to breadboard and test this circuit MG10 In the output salien key filtering section, directly before the power amp it requires BA4560 opamps, of which I have no stock in my parts bin. Of the ones I do have..which would make the best sub.

I believe the slew rate is important here..although I'm not exactly sure what slew rate means??

BA4560 SR — 4.0 — V / µs AV = 1, RL = 2kΩ

opamps that I have
uA741 SR   0.5 V/µs
TL072 High Slew Rate: 13 V/μs (Typical)
NE5532 High Slew Rate: 9 V/μs Typ
OPA2604 High Slew Rate: 25 V/µs
Also found some CD 4049ube (
not an opamp, but sometimes used as an opamp sub in certain pedal circuits)

My initial geuss would be to use a pair of uA741??  Any input appreciated.

Amplifier Discussion / Motor boating..but not the fun kind
« on: December 08, 2017, 03:31:32 AM »
Hi guys,
I've got a AN7523 Panasonic 3w 8v supply chip amp on a breadboard. Works perfectly just as is, although lacking a little sparkle. When a guitar is plugged in isn't doesn't overload, and sound perfectly ok as a jazz amp, if thats your thing..plenty loud enough to practice at home. My plan is to add some some gain/distortio stage..experiment a bit once the amp is stabilised.

I have just added a Tillman preamp to see what that would do, and theres an improve in sound quality.. unfortunately it starts motor boating..slow thud thud thud..about 1.5 secs apart. I looked at the de-couple cap and resistor on the tillman and changed, 51k R to 5k..and the problem went away...when i power from a 9v battery only..if i try and use a wall wart multivoltage supply that i have, motorboating is still there. The problem was the same initially whether i used the battery or wall wart, untill I changed that resistor. Also have tried larger cap values. up to 470uf .no change

When I check the voltage directly from the seems stable, a slight variation by a mv or 2. Measured the breadborad rails when its under load the supply seems to vary by up to half a v..even when guitar just plugged in..also the red led i installed blinks in unison???

I plan on plugging this in so, although the battery works to a point..I ultimately want this issue sorted so i can use use the dc wallwart.

So to summarise, I geuss my questions are...
1. Can anyone suggest how to properly clean up the supply v, or send a link.
2. Why would the amp on its own not have an issue if the dc supply is unstable?
3. Why does adding a small preamp stage cause this issue.
4. Why does the battery fix the issue, but still problem with wall wart?

Tubes and Hybrids / Red Knob Fender Champ 12
« on: August 31, 2010, 07:59:38 PM »
hoping someone can shed some light..
I powered this up the other day, the light comes on but the sound isn't there. There is a really weak signal still coming through - to give an idea how low the volume I can hear the strings being strummed.

I checked the fuse -in tact.
Does this sound like a valve has blown?? I would like to try and fix myself, but have limited knowledge of amps (done pedals and kits) no real experience with mains voltages/valve amps

Any suggestions appreciated on how to tackle this.

Amplifier Discussion / Raytheon JAN6418 sub-miniature pentodes
« on: February 09, 2010, 06:45:22 AM »
Hey there,
I'm new to the forum and Electronics/DIY, but fast becoming addicted. This forum looks like an excellent resource - so much info. Thus far (since Xmas) I have successfully put together a couple of electronics store beginner kits - .5 watt "champ" amplifier (based on Lm386 chip), FM radio tuner, a couple of general guitar gadgets stomp pedals. My latest "conquest" ;) was this little beauty from Oatley electronics here in Australia - which is based around a pair of Raytheon JAN6418 sub-miniaturepentodes - hmm forgot I even ordered it  - a pleasant "Beerloom" from ebay.

Last night I finished it and attached a couple of phonojacks and tried it out with the little .5 watt amp then into an 8" speaker that lives in an epiphone valve junior that has a phono jack attached (EVJ was not turned on) - sound was just distorted mush. Then I tried it through the EVJ powered up and the thing really roared!! There was massive gain boost, and some nice distortion at low levels. A bit noisy as it isn't cased yet and has aligator clips hanging everywhere and the EVJ isn't super quiet itself. But I really like the sounds I was getting:)

I would like to make myself an amp using this tube preamp and matched to a simple IC based amplifier. The documentation for the preamp states...
 "The gain of this preamplifier is adjustable from 0 to 100 times, the output voltage can be up to 5V P-P (1.8V RMS) with a fresh 9V battery, and the widest frequency response extends from 20Hz to 30kHz."

Can anyone suggest an IC amp project that would be suitable-please refer to the specs in the link above for more details-theres a schematic. I'm keen to start making my own circuit boards. Gen Guitar gadgets has some info and a pcb for a Craig Anderton LM383 amp-would this be a good starting point ?? Any suggestions or assistance with this greatly appreciated.

Given that I can't really design or barely read a schematic at this stage (but not for lack of trying), I'm especially keen to find out how to power this amp. At the very least I'm thinking I could figure out how to install the tube pre in a stompbox case but I would love to have it all in the one case. I have an old small behringer bass amp case to house it all.
Any suggestions or assistance with this greatly appreciated.

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