The Mini-A lives!

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Hi forum, I just finished one channel of the Mini, and I'm glad to tell that the PCB's I made realy works.

Then i exposed my own PCB's, I accidently mirrored them, so I had to turn all transistors 180 degrees. This also means the output devices are below the PCB. Not intended, but it works and looks pretty good too. :rolleyes:

The criter runs from a 12V trafo with 66000MFD per rail, that gives about 13 volts per rail including diode losses. The idle current is 2.45A. The DC ofset is about 50mA which could be lower, but I matched or measured nothing before soldering, so I saw that one comming.

I used replacments for all transistors. Output devices are IRF640, others are IRF9630 and BC550B.
 

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Hi Thomas,
glad to see your PCB works out.
Didn´t Grey mention in the original thread the idle current would be 1A with +-12V? And you´re having 2.45A?? How do the TO220 transistors survive that. I guess they get quite hot!?
Can´t you just take the fet´s from original aleph with TO247-case for better thermal resistance?
What´s the sound like ? Are you satisfied with the result ?
OK, enough questions.
And please send some more pics when it´s finished.

Jens
 
Hi Jens, thanks for the interest!

The idle current puzzles me. Grey did indeed mention 1A at 12V. But I don't know how it's measured. I measured mine by running one rail thrugh my amp meter.- Maby that's wrong, anyone?

I did promise a PCB version for TO-247's, but I haven't got the time yet. Might do it this week...

The sound? Pretty sweet in my ears! Exelent voice reproduction. It compares a lot to my Szekers headphone amp. I'll get back to the imageing when I have finished the second channel. I noticed some distortion with high energy program material (rock and electronic music) when turned up. R4 & R5 measured just above 5 volts, a bit on the high side. Q1-Q3 measured both 3,56, a bit on the low side, should be 4 volts as i recall. - Any ideas??

Satisfied? When I'm finished I'm satisfied. :bigeyes: But results so far looks and sounds REAL promising.
 
Hi,
I haven´t build any class a amp before so no experience with that. But the idle current really seems to be too high.
I´ve made a PCB of this scaled down aleph as well but am not so satisfied with the results and wanna definitely include the TO247.
Can send you the layout if I get reasonable results.

Jens
 
Thomas
The amp meter should give an accurate value is it possible that you are measuring AC amps instead of DC?
You can measure DC voltage across the source resistor and use ohms law. The tolerance of source resistor may be very large, though and 0.22 ohm is difficult to measure.
I thought Grey tested this circuit very well, I don't know, you may need to tweak the CCS.
 
Jens, if you can wait one or two days I'll get the site updated and put up boards for to247. I'll keep you informed via e-mail.

Grataku, new measurements confirm old ones. I measured 1.16 volts over source resistors (0.47ohms) 1.16V / 0.47R = 2.46A ! Way too high.

Must I go thrugh all component values again, or do you have any experience I can rely on? :eek:
 
Thomas,
something is got to give, there are not that many components in the circuit.
I haven't played with the mini-A so I cannot pinpoint the problem from here, I am an experimental kind of guy. I think you should try talking to Grey. I am sure that a comparison of a few voltages in the CCS region between what you have and what Grey has will clear the issue right up.
Meanwhile you may want to re-measure all your components. is the device temperature under control? Does it reach a steady state?
 
grataku said:
Thomas,
something is got to give, there are not that many components in the circuit.

Errr, I thourght so. Just being in a lazy mood looking for an easy way out. :D

Meantime I checked the schematic the PCB was done after and all component values, and it all checked out. - At least what I can see. And the temperature: I can't touch the thing after 30 mins. :hot:

A few days ago I e-mailed Jason, asking if he could set me in contact with Grey, because his settings doesn't alow me to e-mail him directly through this board. I haven't got an reply yet.
 
grataku, I changed R11 from 47.5K to 10.0K. R11 sets the bias, which now are 1.5A. I don't know if this is the propper way, and i should change other values too, but it works for now. 1.5A is a good value, because my speakers are 6ohms. The sound seems allright with the new value.
 
grataku said:
thomas,
r11 to 10k seems a little on the low side. Are you using 750ohms for r10? Did you try doubling that to 1.5k?

Aaha, I see! The value used in the original Aleph 3. :D

I just changed one channel to R10=1K5 and R11=47K5. The calculated Amperage is around 1.7 A and is just about right for my 6ohm speakers. Distortion seems to be much less too.
 
Great thread. I got a bug to build Mini-A my self after reading this thread. Final push to start building was, when my JLH class A amp died one week ago after half year of faithful service and several modifications. Output and input transistors were destroyed because of connection error I made in modifying it. So I thought that instead of buying new set of transistors and build one channel over again, I can as well just build a new boards of different amp instead. One day of parts buying and Saturday to solder all together, I thought, is all it will take. Now, make it three days of part scrounging and whole weekend of smelling solder fumes and I am still not finished. What went wrong? Veroboard is a obvious cause of all wasted time. I swore to my self that I would never attempt to make another amp on Veroboard after I finished my JLH amp but here I went again – Mini A is sitting on the Veroboard and will get his first trial today. If its smokes, one thing is clear - I will put it on decent PCB next time.
While there is a Mini-A and several Aleph PCBs already posted, I still wanted to make my own just for fun. Because Mini-A should stand for diminished, economy version of Aleph, I thought that PCB for him should fit the same merits so I tried to make it small - single sided, 7x9 cm board. So tell me what you think – is it maybe to tight or should I leave it as it is?

Argo
 

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frugal-phile™
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I'm not much when it comes to SS (i'm just starting to find my way around tubes and am good with speakers) so bear with me.

Ever since Grey 1st posted the Mini-A i have considered it as the leading candidate for my 1st from scratch SS amp. This thread is strengthening that.

A couple questions:

1/ How much heatsink do i need? The attached picture is a set of 4 i have. For reference they are sitting on a true 2x6. A ballpark calculation of 140 in^2 per sink. Can i live with 1, or do i need 2 per amp? Or is even that not enuff?

2/ i was hoping to use some 18-0-18 225 VA toroids i have, but that would require a choke input filter. What sort of choke makes sense? (as a fall-back i also have some 12-0-12 48 VA toroids)

dave
 
Dave,
your attached picture doesn’t show up.
I think 225 VA toroid should be enough and you don’t essentially need a choke input but you can use a capacitance multiplier. It will save you some big farads and given your 18 volt trafo: 18V x 1.414 = about 25 volts minus diode losses = about 23 volts, then you will burn another four - five volts in capacitance multiplier and you are close to 18 – 19 volts.
One of my friends uses in his JLH amp big air core chokes he wounded himself and the sound was much improved comparing to regular capacitance filter. So I would personally prefer to use choke input if I could afford some more space in my amp case.

Fired up my Mini-A yesterday. At first there was now sound from one channel. Another channel played but distorted heavily. Checked everything and found that a wire from input pot was loose. Fixed that one and powered the amp up again. Now both channels were working but both of them distorted also.
Checked all voltages showed in Aleph’s schematic:
9.2V across constant current source diode;
5.6V across 220 ohm resistor (R4 in Grey's schematic),
5.5V across Q4 (BC547B) and Q5 (or R14, 390 ohm resistor);
0.85 to 0.92 V across 0.33-ohm source resistors of output Fets which makes 2.57 to 2.78 A bias.
DC offset - 1.2V!
It also seems that amp reverses signal polarity ?!?(That I need to check again because it is really strange)
There was audible level of hum coming from speakers but I didn’t bother to concern too much about that issue for start. Much greater problem is the distorted sound. Amp plays almost wonderfully at very low level (I could actually notice that it has very nice soundstage and imaging), but turning the volume up a little bit with musical transients it sounds like either it has only one output device working or power supply caps are all gone bad.
I know, I used slightly different component values and BC transistor instead of ZTX or MPSA, also the output pair is IRFP240 but it shouldn’t make so big difference in my opinion. According to simulations I run before firing the amp up, those values were giving 1.5A bias (what I aimed for) with R11 being 47kohm and R10 1.5kohm (like Thomas’s amp has) with 17 V rails. I ended up with 14,7V for positive rail and - 15,5V for negative rail. Amp has capacitance multiplier power supply, inherited from JLH amp and for JLH it worked like charm, absolutely no hum either. For JHL amp rail voltages were kept equal and about 17 volts, where Mini-A seems to pull more and not equally from that capacitor multiplier.
So am I disappointed? Yes and No. Yes because my initial goal was to get a trouble free, reliable and good sounding amp I can play music all days along (actually my wife listens music much more than me) without spending much money and time on it? No because I got a new toy to play with (I mean that troubleshooting part) during these cold and rainy winter nights.

the saga will continue.....


cheers,
Argo
 
joensd said:
Hi Argo,
I tried to work on my layout and followed yours a little.
Aren´t Q1-Q3 connected wrong?
The sources from all three go together which they shouldn´t.
Should stop now working on it.
It´s late and and one :drink: is enough.

Jens
:att'n: :att'n: :att'n:
Thanks for warning.
You are absolutely right. Q1-Q3 are connected wrong. Another mistake is that Q4 and Q5 are also upside down: it should read B_C_E from left to right not E_B_C. And I am the first victim of that wrong layout. That’s why my amp doesn’t work correctly (it makes me wonder that it hasn’t :bomb: :flame: :RIP: yet and even lets some :note: through). I think the errors happened because I have two Eagle Library folders: one on C and one on D drive and the transistor pin designations are different between those libraries so when I updated Eagle’s Library somewhere in the design process I didn’t paid an attention to the pin designation changes. My err. :ashamed:
I will try to e-mail Jason to remove that PCB layout picture from the thread and post a corrected one later.

Argo
 
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