Modifying the Sonic Impact T-Amp & 41Hz AMP6

[dcheming]

I just picked up one of these little amps the other day from Parts Express and I'm planning on doing some pretty extensive mods to it in pursuit of better sound. I realize there's already plenty of mod sites about this little amp on the web, but the circuit board in mine is a different version so it has a different parts layout. Plus a few people here are using this amp for their K1000 earspeakers so I figured there'd be some interest.


Stock Insides

The audio connectors are very low quality, but what does one expect for $30. Also I found the volume "knob" to be next to useless since it's almost flush with the front panel. So first on the agenda was a recase with much better connectors and a proper vol knob. I'll eventually use a nice wood or metal case, but for now I'm just using an open layout on a scrap piece of wood. It's ugly, but it'll make testing and modding much easier.

New Connector Panel

Test Rig

Next I replaced the vol pot with the stock one from my Xiang Sheng 708B. The power switch in the T-Amp is part of the stock vol pot so if you remove it you'll have to hook up a switch of somekind. Since the Pyramid 5A regulated linear power supply that I'm using already has a power switch on it I just looped the switch wires in the plastic connector together. The new pot sounds better, but the channel balance is bad at low volumes so it was a trade off. An ALPS Blue Velvet will be on the way soon.

Using old 708B vol pot

I took some close up pics of the PCB and I'm going through them to add the arrows and circles to show how to connect things for this version. I'll post those soon. Next I'm doing the input capacitor mod that is detailed here using some Solen 3.0uF caps.

 

[lmilhan]

Looking good!

Just curious, what value are the resistors that you used across the speaker jacks?

 

[cotdt]

For the input capacitors, it's better to use RXF capacitors which are low ESL and low ESR than to use botique capacitors. I had best results with Mundorf RXF. There's also a way to get rid of the capacitor completely but it's a complicated procedure.

Most important are the output inductors. You want to use air foil inductors. This is the mod that has the largest impact on sound.

I find that the T-amp is quite limited in terms of mods because the power supply stinks. I had much better results building the 41Hz AMP5. Mods were easier since you're basically building from stratch, and the AMP5 can drive headphones while the T-Amp requires that the headphones be dual mono. It also has much, much better sound quality than the T-Amp.

 

[dcheming]

Quote:

Originally Posted by lmilhan /img/forum/go_quote.gif Just curious, what value are the resistors that you used across the speaker jacks?

They're the stock 0.15uF mylar caps, but I have some cheap Dayton 0.10uF film and foil caps that I'm going to try later.


Quote:

Originally Posted by cotdt /img/forum/go_quote.gif For the input capacitors, it's better to use RXF capacitors which are low ESL and low ESR than to use botique capacitors. I had best results with Mundorf RXF. There's also a way to get rid of the capacitor completely but it's a complicated procedure. Most important are the output inductors. You want to use air foil inductors. This is the mod that has the largest impact on sound. I find that the T-amp is quite limited in terms of mods because the power supply stinks. I had much better results building the 41Hz AMP5. Mods were easier since you're basically building from stratch, and the AMP5 can drive headphones while the T-Amp requires that the headphones be dual mono. It also has much, much better sound quality than the T-Amp.

I've been wanting to try out those Mundorf RXF caps in a project for a while now, but I not going any fancier than Solens for the T-Amp. I just wanted to see what a cheap Tripath based amp would be like after doing a few mods for as little money as possible. I like the overall sound so I'm going to build something much nicer later like a 41Hz kit, a Charlize, or even a Winsome Labs Mouse amp kit. That's when I'll get all the good stuff.

 

[cotdt]

You probably also want to redo the output filter to make it more aggressive.

 

[Don Quichotte]

Could someone please help me with the DC offset problem I've asked about here?

Also, on the Michael Mardis website linked above there is a mention under the "headphones" heading about the ultrasonic noise which is present even with the normal 4 to 8 ohms load. Can this be a problem with the K1000? I know that:
1. many people have used the t-amp with the K1000, but these parameters like DC offset and, I suppose, ultrasonic noise, vary from unit to unit, so some might have had better luck than me with my particular unit; also, there was a time when there was this rumour that the t-amp can fry one's K1000 and it is not clear to me if there was a good reason behind this and if the problem was the DC leakage or something else (ultrasonic noise?)
2. somebody asked AKG if it's OK to use D class amplifiers with the K1000 and the answer was positive, but the t-amp is not, to my understanding, a typical D class amp (they say it's a T class, right?!)

Thanks!

 

[dcheming]

Finally got around to installing the Solen 3.0uF input caps. I used 30ga wire wrapping wire to make it easier to solder to the pads left over after removing the stock 0.33uF surface-mount caps. The Solens definitely improved the resolution of the T-amp, but I really can't comment on any changes in the bass response due to the small speakers I'm currently using (see sig). I upgraded the stock 0.01uF Mylar output filter caps to some Dayton 0.1uF film/foils, which you can see in the pic too. I also wrapped some copper foil around the four output inductors to help reduce high-frequency emissions a bit.

Since I use my XS 708B as a preamp to control the T-amp I wanted to get rid of the extra vol pot in the signal path, especially since it's low quality. I also wanted to reduce the gain since it's very high so I built a voltage divider network using four NTE 51k resistors. The resolution jumped up a bit as did impact compared to the pot I was previously using.

Here's the area of interest for this version of the T-amp. C4 is the left channel input cap and C3 is the right channel cap.

 

[GAD]

Beautiful pics (especially the one with the schematic drawn in). Fun to watch!

GAD

 

[a shovel]

beautiful. i'm about to rework my tamp in anticipation of K1000 duties :veryevil

 

[dcheming]

Thanks for the compliments guys.

Now I need to figure out something for a case. I want to do something a bit different by using a round case, like 4" PVC cap only much nicer. Need to start scrounging around local stores.

a shovel, what are you planning on doing to your T-amp? Are you going to redesign the output filter too?

 

[dcheming]

A few weeks back I built an AMP6 kit from 41Hz audio and as one can imagine it's definitely an improvement over the T-Amp in all areas. I'm very pleased with how it sounds and a nice case is currently being built to house it. I just swapped the Solens for some M-Caps tonight, but so far I'm not sure if I like them better, which was a surprise. I got some PRP resistors that'll be replacing the stock input resistors as well as some 16ga Neotech wire for the speaker outs.

I'm also building one of these for a friend which will have a Noble volume pot and an input selector switch. Other than those two parts they'll be identical so it'll be interesting to compare them to see if there are any audible differences, if any.

 

[velogreg]

Thanks Dcheming, I am in the beginning of a Amp6 build and those are some sweet pics, very informative. What are you using for power? I was thinking of a Elpac wall wart, though I am still searching for the watt and current requirements for the Amp6. Was it your idea or someone elses recommendation to introduce the caps on the input side.

Any picks with the vol pot wired up?

Greg

 

[dcheming]

For power I'm using a Pyramid PS-9KX which supplies 13.8VDC at 5A. I would get something that can supply at least an amp continuous. I like the Pyramid models because they're easy to find, cheap for the power level, and are versatile due to the binding posts. The PS-3KX can be found for under $20.

As for the input caps, if the audio signal passes through it I use film caps wherever is possible in every single project I build. That's just how I do things, but yeah I read about it too while researching the T-Amp mods.

I haven't even started building the other amp yet since I'm still waiting on some parts, but I'll definitely take pics when I do. My goal is to keep the signal wiring as short as possible so I'm going to mount the selector toggle on the back panel right beside the input jacks. Also the volume pot will be mounted as close as possible to the toggle and will be connected to the front panel by a 1/4" stainless shaft. I'm going to try to use the leads of the input caps to connect the pot to the amp to reduce the amount of wire. For the two status LED's I didn't really want to drill two holes in the front panel so I'm planning on using a single two color LED instead.

 

[velogreg]

Thanks! And please make sure you post pics and details, this is a very worthy project.

dcheming: I noticed the Pyramid PS-9KX pushes 13.8 volts, isn't that a little close to the max a Charlize or Amp six can handle (14V)? I wouldn't want to smoke this new toy. Not much room for fluctuations there.

 

[gychang]

Quote:

Originally Posted by dcheming /img/forum/go_quote.gif As for the input caps, if the audio signal passes through it I use film caps wherever is possible in every single project I build. That's just how I do things, but yeah I read about it too while researching the T-Amp mods.

great photos so a novice like me can follow. thanks,

gychang