[Karu]

Hi Everybody,

I'm planning to use my K-701 with the β22 and have done some calculations. I have made some assumptions, the validity of which I have no idea. Can someone more knowleable check that they at least resemble the reality?

K-701
- Max. input power: 200mW
- Impedance: 62Ω
- Max current: √(200mW * 62Ω) = 56.8mA (I guess this is per can)

Balanced β22 board current draw (4 total)
- From earlier post by AMB, I gather the there is an inherent ~40mA draw by each board.
- Selected quiescent current: 120mA
- Idle current draw per board: 40mA + 120mA = 160mA
- Max current draw per board: 160mA + (56.8mA / 2) = 188.4mA

Power dissipation by β22 board (4 total)
- per β22 amp MOSFET: 8V * (120mA + (56.8mA / 2)) = 1.2W
- per β22 cascade MOSFET: (30V - 8V) * (120mA + (56.8mA / 2)) = 3.3W
- per β22 board: 2 * 1.2W + 2 * 3.3W = 9W

Power dissipation by σ22 board (2 total)
- per σ22 MOSFET: (42V - 30V) * ((2 * 188.4mA) / 2) = 2.26W
- per σ22 rail: 2 * 2.26W = 4.5W
- per σ22 board: 2 * 4.5W = 9W

 

[johnwmclean]

Karu, I can’t help you with the math, I’ll leave that to the more knowledgeable personages, but as for heat dissipation over dual sigma 22’s powering a 4 channel β22 driving K701’s. Wouldn’t even raise a sweat.

 

[Karu]

Thanks johnwmclean. That's my feeling too.

The reason I included it in the calculations is to see whether I understand correctly load's effect on the system. There may be a time later when I want to drive something more hungry.

 

[amb]

Karu, you don't need to add the 56.8mA load current to the quiescent current in order to get the total output stage current. This is because the amp operates in pure class A, the output current through the load comes from the quiescent current (which is much greater than the current through the headphone load).

Other than that, your calculations are mostly correct. Deducting the 56.8mA from all your math causes all your power dissipation numbers to be a bit lower.

 

[askforwhy]

Sorry to bother you guys again.

I am planning using two 2-ch attenuator for my balanced beta22, one for L+ R+, another for L- R-, so I can control the volume individually when listen to two unbalanced headphones with balanced input.
My question is that will it damage the headphone if I don't turn up/down the L+ R+/L- R- attenuator simultaneously?
I just want to make sure before I do something stupid.

 

[digger945]

One balanced 4 channel attenuator would work for both/either.

 

[Karu]

Thanks amb for checking the numbers and clarifying things a bit!

Just out of curiosity. If I attach an efficient speaker to the output, wouldn't I most likely draw a peak current much greater than the quiescent? If I understand correctly, then the amplifier is no longer operating in class A. Where should I add those numbers to the calculations?

A second question in the opposite vein: If I assume, on the other hand, that I will never drive anything more hungry than those K-701s. Shouldn't I be able to reduce the quiescent current to 56.8mA and still be able to enjoy the full range of my cans and stay in class A?

 

[askforwhy]

Quote:

Originally Posted by digger945 /img/forum/go_quote.gif One balanced 4 channel attenuator would work for both/either.

Sure you can.
But when using two different unbalanced headphones(different sensitivity) on beta22, I may want to adjust the volume individually.

 

[amb]

Quote:

Originally Posted by askforwhy /img/forum/go_quote.gif Sorry to bother you guys again. I am planning using two 2-ch attenuator for my balanced beta22, one for L+ R+, another for L- R-, so I can control the volume individually when listen to two unbalanced headphones with balanced input. My question is that will it damage the headphone if I don't turn up/down the L+ R+/L- R- attenuator simultaneously? I just want to make sure before I do something stupid.

Nothing would be damaged, but if you're going to use two pots, you should use one for L+ and L- and the other for R+ and R-. This is to ensure that each stereo channel's hot and cold signals are on the same pot for the best tracking. That said, it 's inconvenient to turn two knobs to change the volume, and if you use regular (especially undetented) pots, then it would be difficult to keep the two stereo sides level-matched.

A 4-gang pot or stepped attenuator is a much better solution.

 

[mattcalf]

Just a quick, 99% hypothetical question that will hopefully become relevant a long way down the track.
And it's probably a question best answered by Jeff at Glass Jar Audio but I'm sure there are many in this thread that can answer it for me.

I have a plan (read: dream) in which I build a Beta22 passive config for a pre-amp as well as a fully balanced build with speaker and headphone out.
Glass Jar Audio only offer three and four board kits, are all the boards the same so in reality: Can I could just order 2x three board kits and build the two and four board amps?

 

[Stevesebastianb]

Have interest in building one, but need to read up on all the details first.

 

[amb]

Quote:

Originally Posted by Karu /img/forum/go_quote.gif Just out of curiosity. If I attach an efficient speaker to the output, wouldn't I most likely draw a peak current much greater than the quiescent? If I understand correctly, then the amplifier is no longer operating in class A. Where should I add those numbers to the calculations?

With an 8Ω speaker load the peak output current could exceed the quiescent current when you turn up the volume, in which case the output stage will flow more than the quiescent current. You don't need to add the output current to the quiescent current, though. The current flowing through the output stage at that point is basically just the output current.

For an amp with a push-pull output stage like the β22, the output stage will drop out of class A operation when the quiescent current is less than half of the peak output current.

Quote:

A second question in the opposite vein: If I assume, on the other hand, that I will never drive anything more hungry than those K-701s. Shouldn't I be able to reduce the quiescent current to 56.8mA and still be able to enjoy the full range of my cans and stay in class A?

You could reduce the quiescent current, but the MOSFETs will be operating at a region of its transfer curve that's less linear. The β22 has a dynamically-cascoded output stage which linearizes the effective transfer curve, but the amp will still sound better when biased to the recommended operating points.

By the way, the minimum quiescent current (Iq) for the amp to stay in class A is 40mA, assuming that you will not make the amp deliver more than 200mW into 62Ω (these are the limits you specified for the K701).

The RMS output current into 62Ω (per channel) is:

Prms = Irms² * R
Irms = √(Prms / R) = √(200mW / 62Ω) = 56.8mA
Ipeak = Irms * 1.4142 = 80mA

The amp must quiesce at least half of Ipeak to remain in class A, therefore Iq should be at least 40mA.
Of course, if you use different headphones, then the power and impedance conditions change and the class A threshold will also change. This is why the default recommended Iq is so high -- to make sure that the amp will run in pure class A for any conceivable headphone load even at very high output levels.

 

[askforwhy]

Quote:

Originally Posted by amb /img/forum/go_quote.gif Nothing would be damaged, but if you're going to use two pots, you should use one for L+ and L- and the other for R+ and R-. This is to ensure that each stereo channel's hot and cold signals are on the same pot for the best tracking. That said, it 's inconvenient to turn two knobs to change the volume, and if you use regular (especially undetented) pots, then it would be difficult to keep the two stereo sides level-matched. A 4-gang pot or stepped attenuator is a much better solution.

Thanks Ti,
I have two pre-matched stepped attenuator, I hope it work fine.

 

[johnwmclean]

Quote:

Originally Posted by mattcalf /img/forum/go_quote.gif Just a quick, 99% hypothetical question that will hopefully become relevant a long way down the track. And it's probably a question best answered by Jeff at Glass Jar Audio but I'm sure there are many in this thread that can answer it for me. I have a plan (read: dream) in which I build a Beta22 passive config for a pre-amp as well as a fully balanced build with speaker and headphone out. Glass Jar Audio only offer three and four board kits, are all the boards the same so in reality: Can I could just order 2x three board kits and build the two and four board amps?

Don’t order the 2 x 3 channel Active Ground kit as one of the boards is used as a ground channel, the difference is a resistor on the ground board (R1) is not used.

What you need to do is order a 2 board passive kit (choose the gain you need) even though Jeff doesn’t advertise this kit he’ll do it, no problems. And a balanced kit, again you will have to nominate your gain with this build also.

 

[Karu]

Thanks amb for a good explanation. I think I got it now.

This will make dimensioning decisions much easier.