I am in the middle of building a class D project. It started off with a 41hz.com amp32, which sounded great but simply wasnt loud enough for the application, for home use it would have been excellent. That hasnt been swapped out and a Amp11, over a 70 watts 0.003 % THD into 4 ohm. Thats should be loud enough and all runs off a couple of SLA batteries.

HI well it tried it and it certainly helped however the amp is still hittign distortion before the we get the SPL we need. Looks like the 12 watt Class-T will get swapped out and th 100 watt will go in it place. Bit worried about the effect this will have on battery life but they are very efficient so hopefully that will help.

That perfect i am gonna swap out the 3.3uf which is meant to go down to 3hz and stick in a .33 uf which has a cut off at 70 something. Do you recon this will allow the guitar amp to go louder without distorting, i recon it should have a possative effect.

Should the input caps of my power amp be chosen to cut bass at the low end, to prevent waffle and conserve amp power? My current input cap should be enough to go down to 3hz but it seem this might be overloading the 10" speaker at extreme volume levels and the digital amp is powering off becuase it has reached it power limits. What would be a sensible cut of frequency for a guitar amp, baring in mind backing track are also played through the unit.

ANy input greatfully recieved.

I just read somewhere that a guitar can put out up to 2.5v peak to peak, surely would means that the preamp  actually need very little gain, in my setup at the moment its got up to 100X gain which seems liek it would be total over kill. I think i need to lower it, this should help the noise floor a lot and reduce the hiss i currently get and higher gain settings.

Actually my gain is even higher because the mixer amp also has 3x gain in it, this doesnt of course take into account the losses for the tone controls which will be significant.

And placement

To be honest i havent built this one as yet, got my handsfull with the own preamp design ( loads of help from Teemuk) and the english channel I have also finished. I did however find some errors on this design which i have now corrected if you want to try it by etching one. I should point out however that this and the tonebender only give a maximum of 10x gain so they might not provide enoguh boost for what you need.

OK well the caps seemed to sort it out to a ceratin degree. No distortion and everythign appears near enough normal. I think i need to change the gain pot from log to lin since as it stands it offers very little control with all the action in the last 1/5 of a turn. My Marshal tone tones dont seem to work very well ethier i think i might need to do a bit of rewiring.

One thing that does confuse me however is the lack of gain. Even with the tone controls removed from the circuit the difference between the gain pot at 47k and at 0 ( I measure it and it was wired correctly) thus changing the gain from 1 to 11 seem really not as much as i would have expected, i mean i can easily force the cirucit into distortion but this is from an ipod at 80% volume. i would have thought much less would have been needed.

I did measure the ac voltage from it using a multimeter and that said i was getting about 0.250 volts, well if that went into the cicuit with 10x gain first stage surely with my t-amp set at a input sensitivy of 2v and a 1.5 gain in the mixer amp i should be well over full volume but this just isnt the case. I might try swapping the opamps once more to see what happen but i find it quite confusing what is going on.

Phil

HI Teemuk,

Thanks a lot words the words of advice, as always you have given some great suggestions.
I already checked for solder bridges so that isnt the issue, i will change the 2 caps around, can't beileve i did that but still. A second more trained set off eyes really helped there. I am 90% sure i have got the jumper in place but will check and i think i will remove the r6, r14 resistors. What do you  mean about the cap polarity if i remove them? Just check everythign is as it should be or somethign else. To be honest i am sure all the cap are in the 'right' direction as in they are the same as my eagle stuff, its just was wrong!! I did actually bypass the tone controls last night and that didnt help.

Do you think as it was with caps in thewrong direction it would be possible to blow the opamps, they seem to go very strange. One minute the cicuit worked a bit other times not at all. I switched to using the cheap ones pretty soon cos i dont want to blow my opa134, they cost a lot.

I will finish off the design by doing as you say and using a dual opamp and removing the reduntant second set off biasing resistors, i just didnt have any OPA2134 at the time and wanted somethign to together quickly. Looking back that might not have been the best option.

Anyway thanks a million you have given me some great ideas about how to go foward.

Thanks a lot,you have really cheered me up.

Right i tried this setup last night and had all sorts of issues. I didnt have a guitar handy so i tried  ipod at near zero volume, gain stages set to minimum, volume minimum. Nada. After turing up the gain and volume i got a output even with the ipod at high volume, messed around for a bit and nothign helped.
New opamps, all sorts, funnily enough swappign out the OPA134 for NE5334 actually helped a bit, not sure why. I was wondering if the 2 cap bleed resistors R6 and R14 might actually drain away the signal if the impediance of the tone controls is too high.

Really not sure what is going on, tried changing the gain in the first section from 3 to 10 that didnt help. Weirdly touching the hot connection on the input jack actually increased the volume a bit, wondering if that would indicate a cap is in reversed. Tested my ground and power voltages and tehy were all fine. Any help would be recieved very very greatfully. Put loads of work in and very dissapointed.

On a brighter not the Class d power amp in the chassis sounded superb, , i have 2 cd input going into my mixer Amp board with a gain of 3 set, that then goes to a pot and into the poweramp, that really transformed the ipod sounds from this thing. Got great volumes and it sounds excellent.

Just to say as it stands this circuit will not work, there is an error in the layout.

I will update if anyone is interested, once i have the thing built. The second gain stage was not biased to the reference volatge, opps

I would have to say on a 12v supply that a Tripath based map would be easily the best option. This is based purely on the fact that they are some of the nicest sounds amps out there at any price and when you factor in the low cost, and tube liek sound you have a real winner. I am current building a guitar amp based on one of www.41hz.com boards and if it is anything like as succesful as my hifi amps based on there kits it should be a huge success.

Hi Joe not as yet, however i have etched the boards and will test it out in a week or so. I am currently building a mobile amp and have got a few preamps to test out in it. I will keep the board posted with my progress.

Phil

Etch this

Tracks, red are jumper wire