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Informative questions before starting β24 build

beta24

Informative questions before starting β24 build

Postby Pieterh » April 22nd, 2017, 2:30 am

Dear Ti, I browsed through your document and the two fora on the Beta 24. It left me with a number of questions. Can I ask these here?

- Which components are most critical for their exact value (except for the matched quads JFETS) ? And is it wise to order more of certain components to have a possibility to select the most accurate value ?

- Did you measure all transistors and capacitors before installing?

- Where to connect the thermal breakers (70 degrees) that can be used with Eps. 24? At the heatsink? I didn't see this on any photo or i didn't recognize?

- Toroid: we need a toroid with a magnetic shielding/encapsulated-potted, but is it an advantage to have this shielding wired such that it can be connected to the chassis ?

- Toroid: I also think about one toroid with two separate channels (rectifier/caps) for the unregulated power suply. So far, I think I will choose for the Class A/B and the toroid with 4*30V secondaries. However, for my information, in order to be able to change a Class A/B design in first instance into a Class A at a later moment, one can use at start a toroid with 8*15V secondaries and total 500VA? For Class A/B one uses for one channel 4 secondaries 15V in series (and make +/- 30V, and use 'the middle ground'), for Class A one uses 2 secondaries in parallel each time and then make +/- 15V? Is there a drawback for sound quality when using this '8*15 V toroid' for the Class A/B instead of the 30V secondaries? The toroid for the regulated section (2*35V, 30VA) is the same for Class A/B and A?

- Toroid: In high end audio it is said that the best amps double their power going from 8 to 4 ohms speaker loads (and again into 2 ohms). This is nearly but not exactly the case with the Beta 24. Can you elaborate on why the B24 is not designed like that? Was it a consideration between costs and dimensions for the toroid and the added value / fact that we never use full power contineously? And If you want to have the Beta 24 be able to output 340 W into 4 ohms for both channels contineously, do you only need to increase the toroid to about 700 VA? or do we also need other changes and larger heat sinks than the Hifi20004U300 chassis?

- I saw that at first you had a star ground and a groundloop breaker, one per channel? Later you got rid of that, and now you only have a 47 Ohms 5W ceramic resistor between RCA input connector ground and the chassis? Pin 1 ground of balanced input is connected to the RCA ground. This implies that only in RCA mode, the Beta input ' - ' is connected to the chassis? In balanced mode, starground is floating? Am I right?
beta24_bal_unbal_sw_sch.png


- Any thoughts on compatibility between B24 and PMC twenty 24 loudspeakers? PMC Twenty 24: Impedance = 6,5 to 8 ohm across freq., 86-89 dB sensitivity, 30-150w power rating. Do you think I need the Zobel network?

- About the zener diodes D7-D10 you write, they need elevated mount because they get hot. Are there any diode heat sinks available and would these extend the lifetime of the diodes? I have seen some picture of a DIY heat sink for a diode on your forum.

- You mention that you made reinforcements to the Hifi20004U300 chassis, Can you tell in short what changes you made to the original chassis? Is that about two alu strips I can see on the bottom side of your chassis? Are there any other reinforcements you made?

- You used two standard alu panels 10mm from HIFI2000 for the front and back. But what additional panels did you use on those front and back panels (those that were processed at FPExpress)? These are 5 mm thick? You also got these at HIFI2000 and had them cut/milled by FPE?

- Not important.....but i haven't seen an option/discussion on a clipping indicator which is rather standard at commercial power amps? I suppose that for home use such an indicator is only for cosmetic purposes. Anyway, is it possible in the B24 ? Any drawbacks?

Thanks in advance! Pieter
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Re: Informative questions before starting B24 build

Postby amb » April 22nd, 2017, 8:26 am

Pieterh wrote:- Which components are most critical for their exact value (except for the matched quads JFETS) ? And is it wise to order more of certain components to have a possibility to select the most accurate value ?

None, but if you really want to be finicky, match the values of R1 and R3; and also R2 and R4. That's it.

- Did you measure all transistors and capacitors before installing?

No, I did not. But I do routinely spot check resistor values.

- Where to connect the thermal breakers (70 degrees) that can be used with Eps. 24? At the heatsink? I didn't see this on any photo or i didn't recognize?

They are mounted on the heatsinks, one per side, between the β24 and σ22 boards toward the top. If you look at the AMB reference β24 power amplifier .fpd files I published in the Files subforum, you'll find the mounting hole for it on the sheet for the main heatsinks.

- Toroid: we need a toroid with a magnetic shielding/encapsulated-potted, but is it an advantage to have this shielding wired such that it can be connected to the chassis ?

It depends on the construction of the transformer. Some have a "screen" wire to be connected to the chassis. The Plitron unit I used does not have it.

- Toroid: I also think about one toroid with two separate channels (rectifier/caps) for the unregulated power suply. So far, I think I will choose for the Class A/B and the toroid with 4*30V secondaries. However, for my information, in order to be able to change a Class A/B design in first instance into a Class A at a later moment, one can use at start a toroid with 8*15V secondaries and total 500VA? For Class A/B one uses for one channel 4 secondaries 15V in series (and make +/- 30V, and use 'the middle ground'), for Class A one uses 2 secondaries in parallel each time and then make +/- 15V?

Oh my. Such a thing will have an unwieldy thick bundle of wires, many of which you won't use...

Is there a drawback for sound quality when using this '8*15 V toroid' for the Class A/B instead of the 30V secondaries?

As long as the voltage is right and there is enough VAs, then there should be no audible difference.

The toroid for the regulated section (2*35V, 30VA) is the same for Class A/B and A?

They are not the same. See the β24 in pure class A post carefully.

- Toroid: In high end audio it is said that the best amps double their power going from 8 to 4 ohms speaker loads (and again into 2 ohms). This is nearly but not exactly the case with the Beta 24. Can you elaborate on why the B24 is not designed like that? Was it a consideration between costs and dimensions for the toroid and the added value / fact that we never use full power contineously? And If you want to have the Beta 24 be able to output 340 W into 4 ohms for both channels contineously, do you only need to increase the toroid to about 700 VA? or do we also need other changes and larger heat sinks than the Hifi20004U300 chassis?

It's not entirely an issue with the transformer. If the power transformer cannot maintain an adequate secondary voltage under all intended loads, then yes, it becomes a factor.

When you drop the load impedance in half, the output current is doubled (assuming the same output voltage). Ideally, this does mean 2x the output power because P = V * I. But there are small resistances in the current path (e.g., output MOSFET "on" resistance, output resistors, wiring resistance, etc.). When you have a current flowing through a resistance, there will be a voltage drop. Even though negative feedback corrects for some of this under normal use, the amplifier will not be able to output any more power beyond where it starts to clip. As you can imagine, doubling the current will also double these voltage drops, leading to the output not being quite 2x into the 4Ω load compared to 8Ω.

Here is a something you probably don't know: Yes, there are commercial amplifiers that tout to have output power that seem to scale up linearly with reduced load impedance. But they do so by playing with the numbers, for example down-rating their 8Ω output power in order to make it 1/2 the 4Ω power. This is for marketing reasons, because it makes the amplifier seem to have an unlimited current output capacity. Obviously that kind of marketing works to convince people of a certainly superiority of that trait, but it's all smoke and mirrors! The fact remains that voltage losses (which cannot be zero in real life) become more pronounced as the output current is increased.

- I saw that at first you had a star ground and a groundloop breaker, one per channel? Later you got rid of that, and now you only have a 47 Ohms 5W ceramic resistor between RCA input connector ground and the chassis?

No, I originally had a single star ground for both channels with a ground loop breaker from there to the chassis. This is still reflected in the diagram shown in the β24 PDF document. For my own build, I later switched to having just a resistor between the signal ground at the input jack and the chassis, for each channel. The ground loop breaker at the star point was eliminated, and the star point is isolated from the chassis. I made this change to cure a (very) faint buzzing noise when the unbalanced input was being used. I don't know whether that buzzing noise was a nature of the particular preamp I had, but the fix seems to have no ill effects with any other sources or preamps (balanced or unbalanced) I connect to it.

Pin 1 ground of balanced input is connected to the RCA ground. This implies that only in RCA mode, the Beta input ' - ' is connected to the chassis? In balanced mode, starground is floating? Am I right?
beta24_bal_unbal_sw_sch.png

No. Don't confuse the triangle ground symbol with chassis ground. The triangle is there only to denote that it should be wired to the signal ground of the amp board. Not chassis.

Here is the ground symbol convention I use for all AMB projects:

Image

- Any thoughts on compatibility between B24 and PMC twenty 24 loudspeakers? PMC Twenty 24: Impedance = 6,5 to 8 ohm across freq., 86-89 dB sensitivity, 30-150w power rating. Do you think I need the Zobel network?

The impedance, sensitivity and power ratings suggest that it will be compatible, but that's not enough information to tell if an output zobel network is necessary. I recommend adding the zobel for a peace of mind.

- About the zener diodes D7-D10 you write, they need elevated mount because they get hot. Are there any diode heat sinks available and would these extend the lifetime of the diodes? I have seen some picture of a DIY heat sink for a diode on your forum.

They do run quite hot, but if mounted as described, are within safe margins. I have no issues with my β24 which was built years ago now, and sometimes the unit is powered up most of the day.

- You mention that you made reinforcements to the Hifi20004U300 chassis, Can you tell in short what changes you made to the original chassis? Is that about two alu strips I can see on the bottom side of your chassis? Are there any other reinforcements you made?

The aluminum plate that carries the power transformers, main rectifiers, solid state relay and the support structure for the bulk capacitors, bleeder resistors and relay form basic reinforcement. The alunimum strips on the bottom helps too. I also added angle brackets on the front and rear panels for the top cover mounting screws.

- You used two standard alu panels 10mm from HIFI2000 for the front and back. But what additional panels did you use on those front and back panels (those that were processed at FPExpress)? These are 5 mm thick? You also got these at HIFI2000 and had them cut/milled by FPE?

Yes, they are milled by FPE. See the .fpd files I linked above.

- Not important.....but i haven't seen an option/discussion on a clipping indicator which is rather standard at commercial power amps? I suppose that for home use such an indicator is only for cosmetic purposes. Anyway, is it possible in the B24 ? Any drawbacks?

The amplifier (not just β24, but just about any linear power amp) does not "know" that it's clipping, so there is no way to do this within the amp circuit itself.

There are different ways to implement this as a separate circuit. A cheap way (and probably the most common) is to light up an LED when the output voltage exceeds a certain threshold. But that is not a true clipping indicator. A more sophisticated solution would be to compare the input and output waveforms and light up the LED when significant distortion is detected. As you might imagine, it's not trivial.

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Re: Informative questions before starting β24 build

Postby Pieterh » April 28th, 2017, 12:05 am

Dear Ti,

Can I check with you 4 new components that replace obsolete components?

B24 Board 2200 uF caps: your panasonic TS-HA (Digikey P6994-ND) is obsolete now, Is this Nichicon OK:
Mouser 647-LGU2A222MELA http://nl.mouser.com/ProductDetail/Nich ... 10wQ%3d%3d
or Digikey: 493-6146-ND

Power supply: 0.01 uF 100 V C0G/NP0: the given # 80-C440C103J1G5CA is obsolete now, is this the right one? Mouser 80-C440C103J1G
http://nl.mouser.com/ProductDetail/KEME ... hswy2Q0%3d
Digikey 399-4451-1-ND

Cz (Zobel): 47nF 80-C322C473J1R5CA is obsolete, is this the right one?
Mouser 80-C322C473K1R http://nl.mouser.com/ProductDetail/KEME ... VKnCPow%3d

Cg (ground loop) 0.1 uF / 250V: is this a good one?
Mouser 810-FK26X7R2E104K http://nl.mouser.com/ProductDetail/TDK/ ... qC5kYwE%3d
or Mouser 81-RDER72E104K2K1H3B
http://nl.mouser.com/ProductDetail/Mura ... 2bEg%3d%3d

kind regards,

Pieter
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Re: Informative questions before starting β24 build

Postby amb » April 28th, 2017, 9:40 am

Pieterh wrote:B24 Board 2200 uF caps: your panasonic TS-HA (Digikey P6994-ND) is obsolete now, Is this Nichicon OK:
Mouser 647-LGU2A222MELA http://nl.mouser.com/ProductDetail/Nich ... 10wQ%3d%3d
or Digikey: 493-6146-ND

Power supply: 0.01 uF 100 V C0G/NP0: the given # 80-C440C103J1G5CA is obsolete now, is this the right one? Mouser 80-C440C103J1G
http://nl.mouser.com/ProductDetail/KEME ... hswy2Q0%3d
Digikey 399-4451-1-ND

Cz (Zobel): 47nF 80-C322C473J1R5CA is obsolete, is this the right one?
Mouser 80-C322C473K1R http://nl.mouser.com/ProductDetail/KEME ... VKnCPow%3d

Yes you can use these.

Cg (ground loop) 0.1 uF / 250V: is this a good one?
Mouser 810-FK26X7R2E104K http://nl.mouser.com/ProductDetail/TDK/ ... qC5kYwE%3d
or Mouser 81-RDER72E104K2K1H3B
http://nl.mouser.com/ProductDetail/Mura ... 2bEg%3d%3d

I recommend something more like the Cornell Dubilier 158X104 or 158X104K, available from Mouser and Digi-Key.

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Re: Informative questions before starting β24 build

Postby poseidonsvoice » April 30th, 2017, 3:35 am

Ti/Pieter,

Hope I can tag along a question if you don't mind as I plan on building a Class A version of the β24.

The output stage unregulated power supply voltage needs to be dropped in half, from ±30V to ±15V, fully loaded for the Class A version. Is there any detriment and/or advantage to building/designing a regulated PS for the output stage? I know the current draw is high, given the 1.4A bias current of the output MOSFETS.

Thanks,
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Informative questions before starting β24 build

Postby amb » April 30th, 2017, 3:49 am

If you add a voltage regulator to the power supply for the output stage, then it must be at least as big (in terms of thermal dissipation) as the amp itself! This is why you rarely see a speaker power amp with regulated output stage supply rails. In fact many power amps run unregulated supply rails for the entire amp.

In the β24 , everything is regulated except the output stage, the output stage is cascoded (very high PSRR), and the topology is fully-differential (rail currents cancel out), therefore any additional regulation would just be wasteful without benefit.

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Re: Informative questions before starting β24 build

Postby poseidonsvoice » April 30th, 2017, 7:56 am

Ti,

Interesting and thanks for the explanation. I was pretty sure that the PSRR of the output stage was high and coupled with the differential topology as well as cascoding would make regulating the output stage voltage rails probably a very moot point.

Regardless, what prompted my query was a neat regulator PS design by "opc" at diyaudio where he implemented quad paralleled LT3081 IC's. Scroll down to "High Power Linear Regulator" for details:

http://www.diyaudio.com/forums/showwiki ... ained_Here

Best,
Anand.
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Informative questions before starting β24 build

Postby amb » April 30th, 2017, 8:30 am

That power supply is totally inadequate for a β24. A σ22 can output much more current, and it is not sufficient.

The Wire is a headphone amp and has very different power requirements than a speaker power amp. The β22 headphone amp is usually powered entirely by σ22(s).
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Re: Informative questions before starting β24 build

Postby poseidonsvoice » April 30th, 2017, 12:40 pm

No.

You looked at the wrong supply. We are not talking about headphone amps! Or their PSU's!

Click on my link above. There is a table of contents on the left. Then go to section 4.4.

It's called "THE WIRE HIGH POWERED LINEAR REGULATOR PSU"

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Re: Informative questions before starting β24 build

Postby amb » April 30th, 2017, 1:16 pm

OK, got it. I still wouldn't use a regulator on the β24 output stages for the following reasons:

1. It provides no benefit given the β24 circuit topology (cascoding, fully-differential)
2. More heat dissipation (in the regulators) to contend with, making the whole amplifier even less efficient
3. Tightly regulated output stage power rails actually reduces transient output power

At any rate, when designing the β24, I have considered all of these factors and what you see is the result. Adding voltage regulators to the output stage would be "gilding the lily". Worse, it throws efficiency, chassis space allocation and build cost to the wind for nothing.

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