Aleph 5 / aleph 30 hybrid ?

amplitone

Member
2001-08-23 6:17 am
I'm thinking of building an Aleph 5 because of its power output, but I also like the performance of the aleph 30 into difficult loads.

Is there a good way to increase the bias current ? What would I have to look out for ?

How would I lower the output impedance on the aleph 5 ?

Would the aleph 3 circuit deliver more power with higher rail voltages, adequate heatsinking and higher power parts ?

Is it safe to increase the value of the input resistor (connected to the unbalanced in) to a higher value at the expense of some high freq. roll-off and lower input gain ?

Thanks for your help !
 
My modified Aleph 5 actualy runs with about 2.5A of bias current.
I changed the source resistors value from 1 to 0.47 ohm, acurately trimming the bias current by an appropriate value for R19.
I used a cermet trimmer, in order to vary the current from 1.4A to 3A, depending on the loudspeaker choice.
The output impedance of the A5 is mostly defined by the NFB factor, and should be about 200mOhm, why do you need to lower this?
If you want to increase the input impedance, you may employ the Volksamp A30 or A60 input stage.

Tortello

[Edited by tortello on 09-24-2001 at 07:03 AM]
 

cp642

Member
2001-08-17 6:00 pm
Well I,m also thinking of building the aleph 5, Circuits shows that the amp's voltage is rated at 34V DC

i was wondering that is it ok to use a 2x24V toroid rated at 9.7A/channel to power it, as the output voltage from these transformers will certainly be less than 34V DC after rectification.

If so will it degrade the amp's performance/operation?
 
Hello cp642,
at a first sight a 2X24V, 480VA PER CHANNEL should be a good choice, at least it seems to me.
I think that 1-1.5V more or less for each rail is not a big issue: the Aleph series is perfectly scalable.
My Aleph5 mono's run with a 500VA, 2X25V per channel, and 2X34Vdc, running at 2.5A (the regulation of the power transformer is about 6%): they run cold and noiseless for many hours.
Tortello
 
amplitone,
If you feel that the Volksamp 30 is superior, but want the power of the Aleph 5, why not build a pair of Volksamp 60s?
cp642,
That transformer sounds a bit scant. A good rule of thumb is to double the power the circuit will need, which in the case of an Aleph 5, would indicate a transformer with a 600VA rating. Note also that the 34V rail would collapse (I'm not counting the drop across the rectifier, which would make things even worse) to perhaps 30V under the current draw.

Grey
 
cp642,
No, for the Aleph 5 (being a stereo chassis) 600VA would be for both.
Were you talking about building monoblocks? If so, I missed it. Sorry. You'd need a minimum of about 300VA for each channel. If you've got two of the transformers you mentioned above, you should be okay. Yes, the voltage will still be a bit low due to losses in the rectifier, but the rail should be firm. The Aleph circuit has enough 'give' in it that you should be able to use it without modification. Output wattage will, of course, be slightly lower, but not enough to be troubling.

Grey
 

amplitone

Member
2001-08-23 6:17 am
I was looking at the datasheet for some transformers and it seems that the VA rating occurs at the safe thermal limit of the device.

When running a transformer at full load, efficiency is lower because the resistance of the windings becomes significant and generates heat.

Running about 40-50% of full load usually gives the highest efficiency for that transformer. Of course, larger transformers are more efficient than smaller ones and also have better regulation (so the output voltages are more constant)

With all the heat that the output devices produce, I don't think we need a hot transformer in there....
 
Hello Amplitone,
I use a 39Kohm resistor, in series with a 50K trimmer.
The power Mosfet source resistor are 0.47 Ohm
In this way I can set the bias current between 1.3A and 3A.
You can reduce the "trimmer" range, if you need only a precision regulation: I prefer a wide range because one of my speaker system, Rogers LS3/5A (ante '86, 16ohm) is a very light load: I prefer this speaker in summer! :)
However, with a narrowes range, you can compensate the mosfet Vgs dispersion: you need only to match them three by three.

Tortello
 
Thanks for the info, tortello.

I am considering a wide range of bias as well (Dynaudio and ProAc mini monitors). What is the math behind the values you chose ? How would I do the calculations for more bias current ? Is the original schematic value of 221K only for 1ohm bias resistors ?

I know 0.47ohms for the bias resistors increases the current through the output mosfets, but how does R19 change the current gain ? Is it a voltage divider into the input of Q6-8 ?

Thanks !
 
Hi Amplitone,
in this circuit, the bias current is set both by the source resistor, both by the "pull-up" resistor, R19.
The source resistor senses the drain (source) current, R19 give a fixed contribution, to avoid an unreasonable voltage drop caused by a source resistance high value, at least I think.
So they are by far contingent.
In the original Aleph 5, the voltage drop trough the source resistor supplies almost all of the required voltage, so the high value for R19.
The net you should consider, for an analytical procedure, is made up by R17-R18, Q5, R19,R20 and R40. I prefer simulations, but for a precision tuning, you should measure the real amplifier.
In my A5 the current may be set between 1.3 and 3 ampere, but usually I prefer 2.5A, to keep the junction temperature below 95°C.
If you dicide to increase the bias current with respect to the original design, remember to scale the "negative clipper" also, changing the R15-R16 ratio.
Tortello
 
Hello Forte,
In the Aleph 5 design, R15, R16 and Q4 form a sort of clipper to protect the active side's hexfets: R15 and R16 fix the maximum value of the current in each hexfet.
I think that this choice is not critical: the current limiter should not operate in "normal" conditions, and the high current capability of the power Hexfet (15A) assures a wide range of values.
In my Aleph 5 -hence with 3 Hexfets per side- biased with 3A -1A per Hexfet- I found a good mix with 220 Ohm for R15 and 150 Ohm for R16. The source resistor is 0.47Ohm.
The clipper operates at about 3.14A per Hexfet, 9.5A for the whole stack.