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β24 monoblocks [[complete]]

beta24

β24 monoblocks [[complete]]

Postby Rabbit » May 11th, 2018, 11:19 am

After having a break from DIY builds to let the attention span and bank balance recover, I've taken on a β24 build. I plan to construct 2 monoblock amplifiers as well as custom stands for them. They will be placed next to my CBT24 speakers. I feel gratitude for those who have documented their builds on this forum. The build threads provide reference information as well as inspiration. Thank you all!

I've build and tested two σ22 boards and two ε24 boards. β24 population is in progress. Pictures now! For higher resolution photos, go here: https://flic.kr/s/aHsmhKJfRK. Scroll down for more text.

Image

Image

Image

Power Supplies
For the large unregulated power supplies, I am going to try out SMPS's. I have acquired four mean well RPS-400-36 supplies, two for each amplifier. They are single rail, so I will use 2 in series to provide (+/-) 36v. These SMPS feature a standby mode and 5v standby power. The reason for trying SMPS's is to test the waters, and to allow my strange casing design to be executed (more on that later.) If they end up being troublesome, I will do another iteration with standard unregulated power supplies. For the σ22s, I picked up 2 r-core xfmrs with dual 35v secondaries. The rating on the eBay listing stated 15va, but the labels on the xfmrs I received say 12va. I hope this will not be a problem.

On/Off control plans:
I intend to step up the standby 5v from the large SMPS's to 12v to power the ε24's. I have also picked up 2x relay modules from eBay that have 4 spdt relays each. I will use 2 of the relays to place the mean well supplies in standby mode. To do this, I will use the normally closed contacts on 2 relays (1 for each SMPS) such that PS-ON is connected to the SMPS ground when the ε24 is 'off'. The other 2 relays will be used in to switch the Line and Neutral of the σ22 xfmr. I have tested the standby feature with a single SMPS, but not yet attempted using 2 in series. I will perform that test soon.

Heatsinks, Enclosure
I picked up some 300mm x 210mm heatsinks and brackets for them from Modushop. As shown in the photos, I am going to use spring steel clips to thermally couple the transistors and caddock resistors to the heatsinks. I've also picked up some Keratherm pads for isolation. I think the spring clips will be nice because they allow a little bit of error in the hole locations. I plan to get some sheet aluminum and bend it to form the remaining part of the enclosure. More on that later...

Timeline
Progress comes in bursts for me. I have a 4 year old and another baby on the way (September 2018.) If I can't get these completed by the due date, it might be a while before I can complete them.

Thanks for following along. Any tips, comments, safety concerns, and constructive criticism are welcome.
Last edited by Rabbit on January 19th, 2019, 2:56 pm, edited 5 times in total.

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Re: β24 monoblocks on custom stands - in progress

Postby SonnyMarrow » May 11th, 2018, 5:22 pm

Looks awesome! Any tips for someone thinking about starting a B24 in the semi-near future?
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Re: β24 monoblocks on custom stands - in progress

Postby lugili » May 16th, 2018, 1:09 am

Thanks for sharing, rabbit. Your spring clips look very interesting, where do you get those? Do you have any experience with them?
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Re: β24 monoblocks on custom stands - in progress

Postby Rabbit » May 16th, 2018, 6:54 am

@lugili the clips are Aavid - https://www.digikey.com/product-detail/en/aavid-thermal-division-of-boyd-corporation/MAX08NG/MAX08NG-ND/1625322 also available on Mouser. This is my first time trying them. I decided to try them to allow some margin of error in hole locations and also to spread out the clamping force on the components. They are surprisingly strong.

@SonnyMarrow I would recommend reading every single β24 build thread. Generous folks on this forum put a lot of effort into sharing their experiences (both good and bad) and helpful tips. It is definitely worthwhile to consume that information.

Current status:
Image

Need to clean the board a bit and install the FETs & caddock resistors. Ordered some metal last night. Progress is good!

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Re: β24 monoblocks on custom stands - in progress

Postby Rabbit » May 30th, 2018, 6:31 pm

I finished the boards and powered them on - no smoke or explosions! One board biased up without issue. The second one is a little bit more finicky. I got .25vdc offset and when dialed to zero it throws the quadcopter out of balance :lol: . I can get the dc offset to .095v; any lower and it gets weird. 22ohm resistor readings are 7.8 and 8.2v. I think that is probably OK, but would like confirmation. Here's the part where my confidence is lower: At this state, R11 is at 83mV and R13 is at 180mV (expected values are ~130mV each) due to VR4 adjustment dialing down the DC offset. That seems a bit unbalanced. Another negative - the SMPS units whine when the amp is idle. I hope it is not audible when it's cased up.

Edit: another SMPS related issue. Without the 22 ohm resistors in place, the SMPS protection triggers upon turn-on. I think this is due to the large capacitors appearing as a short when they are discharged. Drat.

Summary: 1 quadcopter hovering, 1 wobbling slightly. SMPS whines like my 4 year old.

On another note, I am SUPER excited! I've cut some metal, ordered some more metal, and I will report progress (photos) on the casework soon.
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Re: β24 monoblocks on custom stands - in progress

Postby amb » May 31st, 2018, 1:10 am

Try swapping Q1/Q3 (or Q2/Q4, but not both) and see if you have an easier time with the initial setup of the second board.

Yes, you’re probably right about the SMPS.
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Re: β24 monoblocks on custom stands - in progress

Postby Rabbit » May 31st, 2018, 9:01 am

I'll try swapping some JFETS when I get a chance. Thank you for the advice. I purposely left the leads of the JFETs and BJTs un-bent on the bottom side of the board for easy swapping if necessary. (For other components, I bent the leads in the direction of the trace because I saw someone do it on YouTube and it seemed like a good thing to do.)

I'm not going to abandon the SMPS just yet. My first attempt to address the in-rush problem will be to use an in-rush current limiter, bypassed after a few seconds by a relay delay circuit. For the PWM / inductor whine, I'm just going to ignore that problem for now and hope it magically goes away. :roll: :lol:

Late last night, I couldn't resist hooking one channel up to a speaker/input and playing music for a few minutes. Glorious.
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Re: β24 monoblocks on custom stands - in progress

Postby Rabbit » June 1st, 2018, 9:05 am

When I swapped Q2 & Q4, the problem went from 1 side to the other. When I zero the DC offset, one bank of transistors is unable to reach bias before the trimmer runs out. I measure max of 24mV on the .2ohm resistors before the trimmer hits its max. I'll take a look at the IDSS values once I pluck the jfets and figure out how to measure it. I hope it was a simple mix-up while populating the board.
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Re: β24 monoblocks on custom stands - in progress

Postby amb » June 2nd, 2018, 12:49 am

Looks like Q2 and Q4 are not matched well enough.
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Re: β24 monoblocks on custom stands - in progress

Postby Rabbit » June 12th, 2018, 7:49 pm

After a week of travel, I'm back. I've confirmed that I have a transistor matching issue. First, I swapped quads on the board (Q1-4 swap with Q41-44) and ran into the same problem. Then, I swapped the 2 quads of JFETS from my working board to the nonworking board, and it biased up without any issue. That tells me the board is OK but the matching is insufficient.

I measured the trouble quads with my dinky transistor tester, and they showed id of Q1/Q3: .54mA, .55mA; Q2/Q4: .63mA, .61mA; Q41/Q43: .55mA, .53mA; Q42/Q44: .64mA, .62mA. The measurements seem OK, but real world results not so much. I would like to purchase another 2 quads from the AMB shop. (let me know!)

I'm also faced with a mis-delivered package with metal bits for my enclosure. But, I have lots of work to do with the metal that I have, so hopefully it won't represent much of a time delay.

Finally, I've decided to get a small 12v power supply for each monoblock to run the relay circuits etc. Previously, I was using the standby 5v from the large SMPS's and stepping it up, but after adding more relays (to address the overcurrent trip issue), I think it would be best to have a dedicated supply to feed the housekeeping circuitry.

The build is progressing, slowly, and I am very happy with progress I've made so far.
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