Firstwatt F2 Build Log

[hugz]

I've edited the third post of this thread for the 2nd channel.

Quote:

Originally Posted by nightfire /img/forum/go_quote.gif Hmm... so it uses a blocking capacitor I guess then? I couldn't find the schematic but from the F2 service manual it appears the IRFP9240 would be the current source, so yeah, no matching needed. If you're using a blocking cap then DC offset is impossible and not something you have to worry about.

Here is the schematic. I dont really know if it has a blocking cap or not because i'm a tad bit noob. I actually neglected to add the schematic to my build log, so I'll edit one of my earlier posts now.



Quote:

Originally Posted by nightfire /img/forum/go_quote.gif Other things you could check: If you used flux, make sure you cleaned the joints around everything well. Flux is conductive so if you missed a spot with flux remover (particularly around input caps, resistors, etc) that can create noise. There could be an oscillation at play but I doubt it would have that effect. You may want to beef up or shorten your power wires from the capacitor bank. Try bypassing the input cap with a wire and make sure it's not the cause. If you're using zener diodes to dissipate static on the inputs, try disconnecting them. Also try an alternate power supply if you have one.

Thanks. I'll try those things soonish.

By the "input cap" do you mean c1?

Troubleshooting will continue, and listening impressions are still to come.

Yesterday I had the amp on all day in ambient temperature of about 32+ degrees celsius (with no air movement) and the heatsink still didn't get too hot to touch for 10+ seconds. It was getting fairly hot, but i could still hold them for as long as i wanted to.

At the moment the film caps are not too impressive. Since i'm a beginner at this stuff, i accidentally bought electrolytic caps where there should be film caps, so I had to replace them with cheap generic film caps from the local electronics store.

i think i have access to some 1uf paper-in-oil caps. what does everyone think about replacing the generic films with some paper in oil caps? the output caps in my dacah are also paper in oil.

 

[nightfire]

Quote:

Originally Posted by hugz /img/forum/go_quote.gif Here is the schematic. I dont really know if it has a blocking cap or not because i'm a tad bit noob. I actually neglected to add the schematic to my build log, so I'll edit one of my earlier posts now.

Ah, ok. Yeah, the 15,000uF and 1uF bypass (C3/C2) are the DC blocking caps. So you don't have to worry about DC offset frying your speakers.

Quote:

Thanks. I'll try those things soonish. By the "input cap" do you mean c1?

Yep. In this case all it does is isolate the DC bias established by R4/R5 from the input source. Actually those resistors are very low ohmage, so it might not be a good idea to bypass that capacitor. I wouldn't think that kind of current would harm a source, but..

Quote:

Troubleshooting will continue, and listening impressions are still to come. Yesterday I had the amp on all day in ambient temperature of about 32+ degrees celsius (with no air movement) and the heatsink still didn't get too hot to touch for 10+ seconds. It was getting fairly hot, but i could still hold them for as long as i wanted to. At the moment the film caps are not too impressive. Since i'm a beginner at this stuff, i accidentally bought electrolytic caps where there should be film caps, so I had to replace them with cheap generic film caps from the local electronics store. i think i have access to some 1uf paper-in-oil caps. what does everyone think about replacing the generic films with some paper in oil caps? the output caps in my dacah are also paper in oil.

Well, I've only used standard polypropylene caps, nothing too fancy. Personally I like BC components (I used BC2076-ND) but I don't think it matters that much.

A bigger improvement would come in replacing C3. It would be expensive, but changing that electrolytic to a few high uF tantalums would likely be audible.

As for temperature, that sounds perfect. It should run even cooler when you finish the casework.

One other thing you may want to check, if only one channel is crackling, is the pot (P1). It's in the input path, so if it's got flux in it, it'll definitely make noise.

 

[hugz]

Experiments in removing The Buzz

Test: Removing the inputs

Result: No change. Doesn't seem to be related with input, or interconnects

--

Test: Moving the transformer

Result: No noticable change

--

Test: Moving the power supply in relation to the amp

Result: No noticable change

--

Test: Switching the outputs on the transformer (left to right, right to left). This is done because the hum is louder in one channel, so if the transformer is at fault this would presumably cause the noise to become louder in the opposite channel.

Result: No change

--

Test: Put amp behind a power filter

Result: No change

--

Test: Bypassing the input cap with wire

Result: The buzz stops... but so does everything else. without c1 the amp stops working. also, it acquired a HUM while c1 was bypassed

--

Test: *slightly* beefing up the dc input wires (okay i just added another wire in parallel to the positive wire, with alligator clips)

Result: no change

--

I dont know if this is of any significance, but the power supply outputs around 25v I think, which is about what I'd expect it to put out. However, when I connect the power supply to the amp board and take a measurement of the voltage it goes down to a round 22v.

Also, as mentioned above the buzz is louder in one channel than the other.

I dont have any flux cleaner at the moment so that will have to wait

 

[nightfire]

Quote:

Originally Posted by hugz /img/forum/go_quote.gif Experiments in removing The Buzz Test: Removing the inputs Result: No change. Doesn't seem to be related with input, or interconnects -- Test: Moving the transformer Result: No noticable change -- Test: Moving the power supply in relation to the amp Result: No noticable change -- Test: Switching the outputs on the transformer (left to right, right to left). This is done because the hum is louder in one channel, so if the transformer is at fault this would presumably cause the noise to become louder in the opposite channel. Result: No change -- Test: Put amp behind a power filter Result: No change -- Test: Bypassing the input cap with wire Result: The buzz stops... but so does everything else. without c1 the amp stops working. also, it acquired a HUM while c1 was bypassed -- Test: *slightly* beefing up the dc input wires (okay i just added another wire in parallel to the positive wire, with alligator clips) Result: no change -- I dont know if this is of any significance, but the power supply outputs around 25v I think, which is about what I'd expect it to put out. However, when I connect the power supply to the amp board and take a measurement of the voltage it goes down to a round 22v. Also, as mentioned above the buzz is louder in one channel than the other. I dont have any flux cleaner at the moment so that will have to wait

Hmm. Are you sure the - input is grounded to the power supply's negative?

 

[hugz]

Quote:

Originally Posted by nightfire /img/forum/go_quote.gif Hmm. Are you sure the - input is grounded to the power supply's negative?

The power supply and the rca input share the same ground (although they're wired pretty far apart on the ground plane. i can try wiring together if you think it could fix it)

initially we had accidentally wired the rca backwards (positive to ground) and it caused serious problems. on some sources it was really quiet, and on other sources it distorted heavily

 

[nightfire]

Quote:

Originally Posted by hugz /img/forum/go_quote.gif The power supply and the rca input share the same ground (although they're wired pretty far apart on the ground plane. i can try wiring together if you think it could fix it) initially we had accidentally wired the rca backwards (positive to ground) and it caused serious problems. on some sources it was really quiet, and on other sources it distorted heavily

Ah it's probably ok. This is a tough one.

 

[hugz]

Important Note

Unless someone can provide me with a good reason not to do it, I will soon be connecting my F2 up to my headphones (grados and sennheisers). I probably would've done it already, but I'm lending them both to friends

Brief Listening Impressions

Well I'd really like to provide a nice long review of this amp, but I feel pretty uncomfortable doing that without anything fair to compare it against. Thus, for the main listening impressions (which will come later on), I will try to get my hands on some decent tube amps for comparison

So this is what I can say so far: The F2 sounds nice. It has a pleasing soft/warm sound. I dunno how this would compare to a SET because I dont have one on me. So far there isn't anything about the F2 that I can complain about. Bass is improved, but possibly not enough. This is something I'll have to work out later.

I'm not using a preamp because I like simplicity. Without a preamp it is sufficiently loud for most music; however, some quietly recorded albums make me worry a bit because the volume control is near max. I will try it out with a tube preamp when I get the chance

 

[hugz]

Further experiments in finding the noise

I have done the following:

- Reflowing most of the joints
- Repalcing some of the wires with thicker ones, particuarly the ground
- Embarrasingly, the cheapest caps are the film cap bypasses on the dc blocking cap (because i accidentally bought the wrong type in my order so i just had to pick up some no-name ones from the local electronics store), so i wondered if it could be the cause. I removed it to see what would happen.
- I also tried connecting the output to the other side of the output cap (which i think would completely remove it from the chain).

The noise remained through all this testing. Also, I can very much recommend NOT removing the dc blocking cap. Fortunately i used a cheap test-driver when i did that one, because the driver got pushed way out by DC

in other news..

I removed the resistor R13-R15. These resistors are in parallel with the speaker output and provide damping. I removed em for a bit of fun. It's supposed to improve the bass, which I think it has. my speakers still aren't bass monsters though.

When i finally case it all up, I'll possibly put a switch to connect and disconnect these resistors - a kind of bass boost

 

[hugz]

Updates on the noise

Embarrasingly, I think I've found the source of the noise problem. Someone on DIYaudio.com noticed a mistake in my PSU schematic. As a result of the mistake, one of the channels was receiving the right amount of power but it wasn't going through the filtering properly. The other channel was fine.

Unsurprisingly, the problem channel was also the channel which happened to have the louder noise problem. Having fixed the mistake, the noise has now dropped considerably.

I suspect the terrible power in my house (tube amp killng stuff) may be contributing largely to the remaining noise. I'll try out the amp at a friend's house when I have time.

The psu drawing has been replaced so that it no longer reflects this mistake. I'll update the photos too when I have a chance to take new photos of the psu.

A friend is borrowing my headphones, but when i have the chance i'll try out the f2 with them.

 

[hugz]

Headphone amp

Time to start my experiment with headphones.

For starters I'm just connecting one channel of the headphones because I'm not sure if both channels share a common ground or not (and headphone cables *do* share a common ground, but i cant be bothered recabling it right now

Set attenuator to 0 volume and connect grados. Hum, no explosion. So far so good!

Set attenuator to 1 volume and connect grados. MUSIC! no explosion. When the song goes silent the hum becomes apparent. dang.

Repeat with sennheisers, same results.

I played around with putting some resistors in parallel with the output, and around 20 ohms seems pretty decent so far. I may try something lower though to decrease the output (i think).

I'm no EE so I've just been playing around with it, but it seems that putting resistors in parallel reduces the volume output, which i guess probably just hides the hum. I suspect that the hum has always been there but it isn't noticable until hyper sensitive headphones are added. Its not surprising since everything is unshielded and the rcas run right past the transformer.

I'll play around with it some more, but i've got pretty limited access to resistors at the moment

edit: I had previously disconnected R13-R15 which is a resistor bank which runs parallel to the speakers for damping. It seems to be 15ohm unless i screwed something up (three 47 ohm resistors in parallel). Since i was just adding resistors in parallel with the headphones anyway, I decided to reconnect these resistor banks and also add another 47ohm resistor in parallel to my headphone jack (bringing the total to 11.75ohms). This seems pretty acceptable at the moment, but i'll update more when i play around with it some more.

it seems the firstwatts can be used as a headphone amp if you so desire.

 

[varro]

Quote:

Originally Posted by hugz /img/forum/go_quote.gif Updates on the noise Embarrasingly, I think I've found the source of the noise problem. Someone on DIYaudio.com noticed a mistake in my PSU schematic. As a result of the mistake, one of the channels was receiving the right amount of power but it wasn't going through the filtering properly. The other channel was fine. Unsurprisingly, the problem channel was also the channel which happened to have the louder noise problem. Having fixed the mistake, the noise has now dropped considerably. I suspect the terrible power in my house (tube amp killng stuff) may be contributing largely to the remaining noise. I'll try out the amp at a friend's house when I have time. The psu drawing has been replaced so that it no longer reflects this mistake. I'll update the photos too when I have a chance to take new photos of the psu. A friend is borrowing my headphones, but when i have the chance i'll try out the f2 with them.

Have you tried to put a cover to the transformer?

 

[nightfire]

Quote:

Originally Posted by varro /img/forum/go_quote.gif Have you tried to put a cover to the transformer?

I did exactly that and it helped me a lot.

That was 1/16" stainless steel, which is a bitch to work with (compared to aluminum) but made the transformer hum 99.9% inaudible. I can still hear it in a dead silent room if i cover K1000s and put them right against my ears. In normal listening there's nothing.

Without the shield it was quiet audible.

 

[splaz]

Any pics ?

Looks nice but out of my budget

Quote:

Next up, digikey. The price of both of our amps came to $397 usd, but take into account that this is buying lots of extra parts just to be careful (living in australia, I dont want to have to wait/pay for postage from america if one of my mosfets overheat or one of my caps or diodes explode.. plus i'm always happy to have spare parts around for future projects). This comes to $199 USD per amp. AUD$250. Oh, and this doesn't include the transformer and some various small parts that I'd rather buy locally.

I second that. Getting some parts for a few Alien DAC boards I have left. Need to really order extras of the SMD parts as not all can be had here and if the inevitable happens, it falls on the floor, dissapears on the bench, someone eats it

and so on you're sort of stuffed. Plus I often order extra things I think I may need in future and can't get here, quality electrolytics are a prime example, op-amps also. The price breaks really suck me in. "Well I'm already getting 8 and if I order 10 it will actually be cheaper... well... I've already ordered 10, I think I may get 20 for spares and other projects. On second thoughts, I wont be ordering again in a while, better make that 30... hmmm why not take the logical step and make the 50 price break..."

It's a bit worrying but then again your order puts mine to shame.

 

[nightfire]

I highly recommend Digikey's resistor kits (though I haven't tried their SMD ones). I just bought 3 @ $30 each, and they in total include ~1500 1% 1/4 watt, 5 of each value, all the way from 1k to 1M. At least that's one less thing to worry about when working on projects. And obviously especially good for experimenting.

Their capacitor and transistor kits don't seem nearly as useful though.

 

[splaz]

Quote:

Originally Posted by nightfire /img/forum/go_quote.gif I highly recommend Digikey's resistor kits (though I haven't tried their SMD ones). I just bought 3 @ $30 each, and they in total include ~1500 1% 1/4 watt, 5 of each value, all the way from 1k to 1M. At least that's one less thing to worry about when working on projects. And obviously especially good for experimenting. Their capacitor and transistor kits don't seem nearly as useful though.

lalalalalala, can't hear you, lalalalalalalala...

Now you've done it. Something else for me to spend money on...

Might get them later actually. My wallet would keel over if I add that in.