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Transformer sizing and grounding

beta22

Transformer sizing and grounding

Postby johan851 » October 29th, 2019, 12:38 pm

I'm thinking about a β22 build for dual-duty headphone and speaker use. A while back I was looking at a much bigger balanced setup for bigger speakers, but I've scaled down my speakers recently and want to scale down this build as well. I'll replace my current M^3 + Gainclone setup with this.

I'm planning on a two-board build with one σ22 per channel. The speakers will be these or something similar for nearfield use. The set I linked has drivers rated at 86.5dB/W(m). Yes, it's a little sad to be reduced to 3" drivers, but such is my life. I'll add a line out to the build so I can support a subwoofer too.

I'm trying to fit everything into a smaller chassis, roughly 8"x8"x3". I had thought I could power both β22 boards with a single σ22, allowing me to also fit the transformer in the σ22 case, but doing some rough thermal calculations for onboard heatsinks it seems the σ22 would be a significant bottleneck. To address that, I'll move the transformer to its own case and stash it somewhere with a long-ish connecting cord. At least that'll keep any interference far from the amp.

My questions:
1) Using 2" onboard heatsinks and a shared transformer for the σ22s, with +/- 27V output, I'm thinking a model around 200VA seems right. Avel Lindberg has a 230VA unit with 25V secondaries. Is that overkill?
2) The transformer chassis should be connected to AC ground, the β22 chassis should be connected to input ground, but how should I ground the σ22 chassis in this configuration?
3) Should the ground wire for the speaker binding posts return to that channel's respective σ22 ground terminal? For the headphone jack, I imagine I can just pick one of the σ22 ground return wires?
4) Is a gain of 8x suitable, or is a gain of 11x preferable? The source will be my y2.

Thanks!
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Re: Transformer sizing and grounding

Postby amb » October 29th, 2019, 11:03 pm

You might want to Search this forum about β22 being a "light duty" speaker amplifier for more discussion on this subject.

johan851 wrote:1) Using 2" onboard heatsinks and a shared transformer for the σ22s, with +/- 27V output, I'm thinking a model around 200VA seems right. Avel Lindberg has a 230VA unit with 25V secondaries. Is that overkill?

Maybe. Search this forum about β22 being a "light duty" speaker amplifier for more discussions related to this subject. Your use of efficient speakers in a near field probably qualifies as "light duty".

2) The transformer chassis should be connected to AC ground, the β22 chassis should be connected to input ground, but how should I ground the σ22 chassis in this configuration?

If I have it right, you're talking about a 3-chassis configuration:

chassis #1: Power transformer only
chassis #2: Two σ22s
chassis #3: Two β22s

If so, chassis #2 doesn't have mains AC voltages in it. In this case, you can just connect it to σ22 grounds.

3) Should the ground wire for the speaker binding posts return to that channel's respective σ22 ground terminal? For the headphone jack, I imagine I can just pick one of the σ22 ground return wires?

Yes and yes.

4) Is a gain of 8x suitable, or is a gain of 11x preferable? The source will be my y2.

This depends on your speakers' efficiency. Those speakers you linked to use efficient Fostex drivers, so 8x gain is probably enough, especially for near-field use.
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Re: Transformer sizing and grounding

Postby johan851 » October 30th, 2019, 2:04 pm

Thanks for the answers. I based my transformer size on a few other threads that typically recommended around 120VA per channel for speaker use. The sigma22 page also recommends the following:
If the σ22 will be supplying power to a β22 amplifier intended to drive speakers, it is recommended that one σ22 board be used per channel, each board supplied by its own 100VA transformer. A similar recommendation applies for other power amplifiers.

Space for the transformer isn't really lacking given the dedicated chassis, and I imagine a bigger transformer can't hurt because the regulation should be better. I'm mostly wondering if I'm far off the mark given thermal constraints, my "light" application, and the impact (if any) of sharing the transformer across both channels. The 230VA transformer is spec'd for 4.60 per rail (2.3A per channel if shared) and that's beyond the thermal limit for continuous use with onboard heatsinks, but I'm unsure how to account for peak current demands.

Two related questions. Is there any value in increasing C5/C6 on the σ22 boards since these will be used for speakers? PSRR appears to be excellent for both the σ22 and β22, so I imagine the capacitors would have to be increased to impractical sizes to have any impact. Second, should I worry about inrush current if I locate the power switch between the transformer output and the σ22 input?
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Re: Transformer sizing and grounding

Postby amb » October 30th, 2019, 4:52 pm

There is no real benefit to increasing the capacitance of σ22’s C5/C6. All that would do is increase the turn on surge current and stress the rectifiers, transformer and power switch.

I recommend putting the power switch on the primary side of the transformer, so that when the power is off, the transformer is also off.
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