[moonboy403]

Quote:

Originally Posted by Beefy /img/forum/go_quote.gif I used 33 ohm output resistors for 32 ohms AT-ESW9s. Works great as a balance between noise/gain reduction and damping for these sensitive phones. But in all honesty, as much as I love the MiniMax I don't think that it is even remotely suitable for IEMs. Too high a gain, too high a noise floor. Even higher resistances or impedance adapters might help with IEMs, but your damping factor will drop off a cliff. No, I really doubt that is all it is.

I'm using a UE impedance adapter which is said to be around 100 ohms. All hiss and noise are eliminated. While SQ degraded a bit, I still think my JH13 sounds very good the MiniMax.

Beefy, seeing that you have the Superfi 5, can you test it out on your MiniMax to see if there's any hiss? Also, are there any specific brand of output resistors I should be looking at? A link to which resistors I should be buying would be nice (so I only have to them to cart and buy them).

The reason why I wanna keep the MiniMax is because of its capability in driving wide range of headphones. I already have a speaker amp dedicated to the K1000, so if I buy a portable amp for the JH13, I'll have to buy yet another amp for my other headpones.

Thanks

 

[Beefy]

Quote:

Originally Posted by moonboy403 /img/forum/go_quote.gif Beefy, seeing that you have the Superfi 5, can you test it out on your MiniMax to see if there's any hiss?

I previously have. I normally listen at fairly low volumes, and the noise floor was just too high to be able to enjoy it at all.

Quote:

The reason why I wanna keep the MiniMax is because of its capability in driving wide range of headphones. I already have a speaker amp dedicated to the K1000, so if I buy a portable amp for the JH13, I'll have to buy yet another amp for my other headpones.

And it is worth keeping. But different/larger in-line resistors aren't going to be any better than the impedance adapter you are using now.

Perhaps you could consider making yourself something in a cable adaptor at intermediate values between 33 and 100 ohm? The default boutique suggestion for the Max amps is Kiwame or KOA SPR2. Beezar still stocks 47R KOA which might be a good place to start.

 

[iamthecheese]

Well it turns out one of the ground pins on the volume pot wasn't making good contact. So after that was fixed everything works and sounds great.

I'm gunna case it up after class today then post some pics!

 

[wierddemon]

Quote:

Originally Posted by tomb /img/forum/go_quote.gif I would recommend that you not mess with the DB's. No offense, but if you have trouble keeping mV straight from V, you could fry the amp in no time. Seriously (not that the above was a total joke), practice a little more taking readings, swapping tubes and adjusting the tubes until you are completely confident what you are reading and how the changes occur relative to adjusting the trimmers.

Hmmm... alright, I guess I'll just hope that the DB is set properly at this point.

I get that I am in the DIY form and that everyone here is more advanced than me, but I did specifically say that I was just getting into this stuff and trying to learn.

 

[tomb]

Quote:

Originally Posted by wierddemon /img/forum/go_quote.gif Hmmm... alright, I guess I'll just hope that the DB is set properly at this point. I get that I am in the DIY form and that everyone here is more advanced than me, but I did specifically say that I was just getting into this stuff and trying to learn.

Yes, I sort of figured that. However, "learning" with the Class A bias on a Diamond Buffer is the sort of first lesson that can be painful (to your wallet) and destructive. We don't even recommend the MiniMAX as a first build, and only a second with some qualification.

Build something - perhaps a CMoy, learn a little more about making meter readings and then perhaps BJT's and thermal runaway. Bias the tubes all you want - the amp is built with those adjustments in mind without having to take one apart. DB bias is different.

I'm sorry if that's not taken as constructive advice, either, but I would never suggest to someone that they could take a MiniMAX apart and adjust the DB bias without having built it or without being a somewhat experienced DIY-er.

 

[iamthecheese]

First before I post the pics I just wanted to say that Thomas(tomb) of Beezar was an INCREDIBLE help and showed great patience with my build problems. There is NO way I could have gotten to where I am with the amp without him.

Now, pics as promised:
Flash:

No Flash(I love the LEDs under the tubes):

 

[tomb]

Thanks, iamthecheese - those are very kind comments. However, you deserve the bulk of the credit. There are many who would have given up long ago in the face of the great work and trouble-shooting that you did. Thanks for sticking with it and discovering the problem yourself in the end!!

Pics look great!

 

[tomb]

See how mixed up this got me? Sorry - wrong thread!

(EDIT: Starving Student kit sales w/o Priority Mail shipping)

 

[phangtonpower]

Just received the Minimax from whiplash, and couldn't be happier, but I have a question about biasing the power supply. I live in Japan and the voltage here is 100v. When I bias the amp with the tubes in it the highest I can get it is 23.9VDC. Seems kinda low, probably the low voltage here in Japan? When the tubes are out, I get about 26.9VDC, which seems closer to the recommended optimum VDC. So I guess my question is, should I be taking the readings with the tubes in or out?

 

[rds]

What do you mean by "biasing the power suppy"?

Are you measuring voltage using the red, blue and white tip jacks on the back of the case? Which trimmer are you adjusting?

To me it sounds like you're adjusting the tube bias, which should be at 13.5V (between red and black, and between blue and black) with the tubes in and warmed up.

 

[phangtonpower]

Sorry I didn't mean biasing, just setting the Power Supply. The probes are on GND and V+, and trimming it at RR3. I have the tubes biased at 12VDC since the PS is at 23.9VDC.

 

[rds]

Ok gotcha. To answer your question the reading you get with the tubes in is the "real" voltage under load. Although 24V isn't the "optimum" voltage, imo it is not something to worry about.

The transformer you have is rated for (NA) 120V AC, so that is the cause of your lower regulated voltage. You could look for Japanese 24 VAC (750mA or higher) wallwart with a 2.1 x 5.5mm barrel plug connector. I don't know of any offhand, but I would imagine they are available from local electronic parts stores.

 

[phangtonpower]

Cool thanks. That's what I figured. Just wanted to clear that up.
Nice build by the way

Edit: I live out in the country side, so it is a little difficult.

 

[tomb]

Quote:

Originally Posted by phangtonpower /img/forum/go_quote.gif Cool thanks. That's what I figured. Just wanted to clear that up. Nice build by the way Edit: I live out in the country side, so it is a little difficult.

You might try going ahead and biasing the tubes up to 13 or 14 anyway - and see how they sound. 12FM6's especially like a bit higher voltage on the plates. Depending on the headphones and source, I doubt that you'll end up clipping - with popular music, anyway. I regularly bias my 12FM6's to 15V, but that's with 27VDC on the power supply.

 

[phangtonpower]

Thanks Tomb and RDS.

Don't ask me how, but I found a 24V 2.08A transformer and have everything set perfect

Kinda new to biasing tubes, and was wondering what the advantage of having higher voltage on the tubes are if I listen at lower levels? Right now I have my amp in between 9 and 10, which is probably my average listening levels. Sometimes I'll crank to 12, but not often. I'm using 12AE6's biased to 14 with my HD650's. Source is a gamma y2. Is it to give headphones a higher voltage swing even at lower levels? Sorry for the Newb question