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Aikido 24V headamp - Page 2

post #16 of 570
Listening impression with K701:

Source: PC>>monica DAC>> either the aikido or millet max mosfet.

The aikido gives a "fuller" sound - just a bigger richer sound. Put on your k701s with your max and put your hands over the cans and squash them into your head - putting the divers closer to your ears - thats kind of what its like. There seems to be a little more detail as well - cymbal/ hi-hat hits are a bit better defined and are a little more sparkly. No harshness or sibilance from either amp.

Now the differences aren't huge, but if you were starting out and were thinking of building a lower voltage valve amp, this would be what I'd go for right now. You also get a full function linestage thrown in as well.

Having said all that the DC at start up issue does need to get sorted. I neglected to say that this only occurs on the headphone outputs, not the linestage outputs.

If I get a chance later I'll repeat the exercise with the Hd650s. If I was to be asked for an explanation, I'd say that the aikido had more current available to effortlessly drive the k701s.

Fran
post #17 of 570
Thanks yet again, fran, very helpful.

I don't even have R17 on my BOM...wonder how I missed that. I haven't seen any values for it suggested anywhere, either (nm, just saw it in the original article. no explanation, though). I guess 5k will do. What is the hole spacing on the heatsink you specify? Looks like ~0.75in. Since the 'sink's not getting warm, maybe I'll tweak R15 down a bit further to up the current.

Thanks for the heads-up on the LDR based volume control. I was debating between three different options (JB's stepper, joshua tree attenuator, and TKD pot), but it looks like I'll have to read up on this one and possibly add it to the list for consideration (the TKD pot was in the lead).

-edit- Just saw your impressions...looks nice! The max is a very good amp, and differences between it and higher end amps do tend to be a bit more on the subtle side, usually in refinement/smoothness as well as detail retrieval and tone.

I look forward to your impressions with the HD650.
post #18 of 570
I have a BOM for mouser, I don't how to get it to you, but I'm sure I could convert to excel or something of that makes life easier.

Tellin' ya, get the optivol. Well worth it - you'll need a 1k8 resistor since you'll be running it off 24V rather than the designed for 30V. (It needs about 5.5V, and draws 13mA, so you can run it off pretty much any lower voltage supply just by changing the resistor using an online ohms law calculator). Heres a guarantee for you: If you don't like it, I'll swap it for a stepped pot that I bought from ebay. If you think thats generous, well think of it this way: I think its better than the stepped pot, so I have a useless stepped pot here - if you don't like the optivol I can only gain!

Fran
post #19 of 570
I've decided to give the optivol a shot, it's not very expensive. I've got the BOM more or less complete, but thank you for offering yours I've also decided to use a tread and run it at ~25.2v. I'm a big fan of both the TKD pot and the Joshua Tree, but I like to try something new now and again .

-edit- I'm going to use a 24VAC wallwart transformer to the TREAD. I'm trying to decide on a heatsink to use on the LM317, but I can't seem to find one that is large enough and will still fit to handle the heat (based on Tangents calculator). You said in an earlier post the heatsink on yours barely gets warm...are you using the recommended part or a different one?
post #20 of 570
Get an extra couple of caps and set the tread to 25.2 and then use something like 10uF/15R/220uF for the B+. That'll give more filtering and drop it to 24V. Run the optivol off this supply. If you have a choke use that instead of the resistor.

Fran


EDIT: the heatsink I'm using is this one from mouser #532-513102B25 Its about 40mm high (1.5 inch) and one inch mounting spacing - which is what is specified in the literature. The cost between the different heights is small and even at 2" high they would still be lower than the tops of the tubes. Thats only an issue if you want the tops of the tubes to stick out the top of the case. But, yes, the 1.5" ones hardly get warm - I've had this here turned on playing all day and just opened the top just now to see and the heatsinks are just a little warmer than body temperature, wouldn't say more than 45 degC. As a whole this is not an amp that gets all that warm, even the tubes don't throw off that much heat (area over the heatsink is warmer than area over the tubes).

To increase the idle current he told me to reduce R17, which was at 10K - so I just paralleled another 10k each side.

BTW, there was an updated user manual for the 24V aikido on broskies site, its about 3 blogs back or so. Contains a good few more details that weren't included in the earlier one. Another thing: theres no R13 on the board and I think its R20 and 21 are the same value.

Further disclaimer: the max amp I comparing above has a stepped attenuator in it whereas the aikido has an optivol. that may make a difference to the sound too. Also, just in case someone thinks I'm a shill for ska audio, I have no connection with them only bought 2 optivols and think they're great, and in fact bought another 2 tonight for the aformentioned max and another preamp thats in the pipework.
post #21 of 570
I figured I'd run the whole thing at 25.2 (JB said he'd tried a 25.6v supply and said it sounded slightly better), though I suppose the extra filtering could help.

-edit- I took a look at your pics again. Do you have a part number for the heatsink used on the tread? That's the last piece for my BOM, then I'm ready to go.
post #22 of 570
simultaneous posting: see the edit to my last post....


Fran
post #23 of 570
Just to be clear: You're using the same heatsink on the aikido and on the tread?

Quote:
Originally Posted by fran View Post
Further disclaimer: the max amp I comparing above has a stepped attenuator in it whereas the aikido has an optivol. that may make a difference to the sound too. Also, just in case someone thinks I'm a shill for ska audio, I have no connection with them only bought 2 optivols and think they're great, and in fact bought another 2 tonight for the aformentioned max and another preamp thats in the pipework.
I hope even after the optivol upgrade for the Max that the aikido will still be superior. I'd like it to be on par with a single-ended B22, but that could be asking a bit for only spending about $450 (what my BOM totals, including casing, wiring, etc.).
post #24 of 570
The heatsink on my tread is actually a bit smaller - it was a spare I had in my parts stash. One of the same will do, but on the tread it hangs off the side of the PCB so you may need to cutoff the legs on the heatsink. The whole thing when running does not draw that much current, <500mA in total IIRC.

$450 sounds dear, but I suppose when you add it all up....

Fran
post #25 of 570
Haha, that's what putting blackgates and multicap rtx caps in your BOM will do I'm saving the boutique resistors for later...just using PRP for now.
post #26 of 570
Actually the tread heatsink that I'm using right now gets a good bit hotter than the ones on the board - so just ordering another one like the others (that little bit bigger) would probably be a good thing. I might put a bigger one on mine. when you are putting the tread together, either premount the 317 on the heatsink and then solder it in, or else leave the legs on the 317 longish so that you can fold it back a bit to allow some clearance for the heatsink.

Another thing. I plugged in some disposable 8 ohm headphones (from the 70's!) and turned on the amp from cold. Got essentially no DC surge even as the caps charged.

I plugged in the HD650s and did the same. Voltage went up to about 1v for a second and then dropped rapidly. No damage to the cans.

I'll try this with the k701 as well.

I would really like to be able to leave the cans plugged in and not have to remember to unplug them each time. From that observation it would seem that while the DC offset does rise, it is at a low low current so it might not matter.

Fran
post #27 of 570
I'll probably add the resistor to short tip and ring to ground when there's no headphone plugged in.

Have you had a chance to listen to the amp with the 650's? It sounds like it pairs very well with the K701's and the K501's.

I'll be ordering the bulk of the parts on Monday, so hopefully I'll be able to start by next weekend. I should be able to finish the balanced B22 I'm working on this weekend, so my schedule will be more or less clear.
post #28 of 570
Do you think I can use non-polar blackgates in the C4 position? I have polar blackgates in my BOM, but I'd prefer to use the NX series if it'd work. It seems to me you shouldn't need polarity in that position.
post #29 of 570
Quote:
Originally Posted by fierce_freak View Post
Do you think I can use non-polar blackgates in the C4 position? I have polar blackgates in my BOM, but I'd prefer to use the NX series if it'd work. It seems to me you shouldn't need polarity in that position.
Polarity of electrolytic caps is more like a constraint than advantage or something you would need. Non-polars will do fine. For a practical proof check all those Millett hybrid builds with BG NX or Nichicon ES at the very same position.
post #30 of 570
Great, thanks a lot kvant. It seemed logical that I could use a nonpolar there, but I wanted to make sure.

-edit- Could the same be said for C11? It seems the same logic would roughly apply.
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