[matt_garman]

Hi all,

I've got an SDS-470 I built, being powered with a Connex SMPS800R power supply (i.e., not the standard linear power supply that Tom/Class D Audio uses).

It generally works great, except, I noticed that when I turn it off, there's a "thud" or "thump" that comes out of the speakers.  This happens maybe a couple seconds after the power is disconnected.  I assume this is the capacitors of the PSU discharging.  But I have a second, complete pre-build SDS-470 (which uses the stock linear PSU), and it doesn't do this.  Note that when I turn the amp on, there is no such noise.  Why would the SMPS do this but the linear PSU not?  Note that I've never tried this particular SDS-470 board with a linear PSU, so I suppose it could be a problem with the amp board itself as well.

Some basic web searching on this kind of issues suggests it may not be a real problem, i.e. OK for my speakers.  Can anyone confirm this?  Even if it's OK for my speakers, it's disconcerting to hear it.

Thoughts?

Thanks,
Matt

[earplay]

Quick question: I've got a TKD stepped attenuator. Would that work instead of a single-source pre-amp?

[john dozier]

Depends on its impedence. If it is a 10k you would probably be ok. 20k is chancy, but you can try. BTW I too have several of those-IMHO the best stepped attenutor out there. Regards

[srb]

Quick question: I've got a TKD stepped attenuator. Would that work instead of a single-source pre-amp?

It would no different than a number of integrated amps that don't have a preamp gain stage but use a passive volume pot or stepped attenuator.
 
There are several things to be aware of when using a passive "preamp":
 
1. Interconnects between the preamp and power amp should be as short as possible, and the internal volume pot would be the best possible scenario in that respect. 
 
2. An active preamp provides impedance matching between the source and the power amp.  Ideally the output impedance to input impedance would have a ratio of 1:10 or greater.  Sources with a high output impedance may not match up as well.  Note: The Class D Audio CDA series have an input impedance of 7K Ohm while the SDS series have a higher 47K Ohm input impedance.
 
3. If you have really low sensitivity speakers and/or low output voltage from your source, you may not have enough gain and might need an active preamp with additional gain.
 
Steve

[earplay]

Thanks john and SRB! I will be connecting to a dac as a source. I couldn't find any info on the output impedance for a Music Hall 25.2 cdp.

I imagine that the output impedance for various dac's is similar. Is that right?

I also assume it would work well with a passive setup. Is that right?

[srb]

I imagine that the output impedance for various dac's is similar. Is that right?

Most of them have pretty low output impedances, such as Benchmark DAC 1 = 30 Ohms, Cambridge Audio DacMagic Plus = <50 Ohms, Simaudio Moon 300D = 100 Ohms, etc.  I have seen some DACs with higher output impedances like 5K Ohm, and those DACs would be a better match with the SDS series 47K Ohm input impedance than the CDA series 7K Ohm input impedance.
 
I couldn't find any reference to the output impedance of the Music Hall DACs, but you will likely be just fine.
 
Steve

[earplay]

Now that I've got the attenuator issue settled for now, I am wondering about transformers.

I've got a few transformers. One from a tubed Cary pre-amp. Could that be used to build a Class D brand amp or should I buy one of their transformers? (I know I am not giving much to go on here, but the answer may be a simple one. I don't know.)

[srb]

I've got a few transformers. One from a tubed Cary pre-amp. Could that be used to build a Class D brand amp or should I buy one of their transformers?

It depends on the amplifier board.  You will need a transformer with sufficient VA rating of course, then the particular amplifier board you have will determine the AC voltage of the dual secondary windings that will be required.
 
Steve

[JohnR]

A power transformer from a tubed preamp won't have anything like the voltages (or current rating) needed.

[srb]

A power transformer from a tubed preamp won't have anything like the voltages (or current rating) needed.

I was discounting the Cary transformer right off the bat, but he said he had other transformers.  But having one with the proper secondary voltages and capacity for a particular board might be a stroke of luck.  The CDA and SDS amps will require transformers with dual AC secondaries from the 21V - 28V range to the 44V - 51V range and 300VA to 500VA capacity depending on which Class D Audio amplifier board.
 
Steve

[earplay]

Thanks guys. I'll see if I can find out the specs on the transformers. I have three. Two are from an old Cary power amp, the first one they made. And I mean, this amp is probably the first one they made, i.e a prototype. 

I'm junking them both. Too much trouble. I'm planning to use the pre's case for the Class D. I've got some good caps in the power amp with very few hours on them. I'll salvage them along with the pre's case and it's connectors.

At the moment, I'm waiting to hear from an AC'er if he wants the transformers. If he doesn't, let me know. I'll be giving them away so packing and shipping is all I'll want. The models are Sweet 807 (one EI laminated core power transformer, one ultralinear output transformer) and SLP 70 (no specs in the manual or on the transformer, custom manufactured by Peter Dahl).

[mikeeastman]

Does anyone know what the power draw of the SDS amps is or where I can find it? My amp is 12 volt, but I assume the amp draw would be the same for a/c and d/c amps .

Thanks, Mike

[mboxler]

Does anyone know what the power draw of the SDS amps is or where I can find it? My amp is 12 volt, but I assume the amp draw would be the same for a/c and d/c amps .

Thanks, Mike

Mike..

Do you mean the power supply is the 12 volt DC PS from Class D Audio?  All the amps are DC power in, AC signal in, amplified AC signal out.  Sorry if I misread your question.

Mike

[mikeeastman]

Sorry I wasn't clear. Yes 12 volt power supply.

  Mike

[justd]

Hi all,

I've got an SDS-470 I built, being powered with a Connex SMPS800R power supply (i.e., not the standard linear power supply that Tom/Class D Audio uses).

It generally works great, except, I noticed that when I turn it off, there's a "thud" or "thump" that comes out of the speakers.  This happens maybe a couple seconds after the power is disconnected.  I assume this is the capacitors of the PSU discharging.  But I have a second, complete pre-build SDS-470 (which uses the stock linear PSU), and it doesn't do this.  Note that when I turn the amp on, there is no such noise.  Why would the SMPS do this but the linear PSU not?  Note that I've never tried this particular SDS-470 board with a linear PSU, so I suppose it could be a problem with the amp board itself as well.

Some basic web searching on this kind of issues suggests it may not be a real problem, i.e. OK for my speakers.  Can anyone confirm this?  Even if it's OK for my speakers, it's disconcerting to hear it.

Thoughts?

Thanks,
Matt

Try asking Tom and christi

[justd]

I think this has been discussed earlier, but it is not in me right now to scan the 172 pages right now. I want to know if there is a sound quality difference between the CDA and the SDS amps. Or is it just a matter of the balanced inputs and the volume pots.
My pre-pro is unbalanced right now so the balanced inputs will not make any difference to me right now.

Hey matt, I read the christi's reply to your query. But I could not understand it. Could you explain it for the benefit for othe forum members.

Thanks

[steve k]

IMO, the SDS has a little more depth to the soundstage and more 3D quality to the imaging. YMMV.
Steve

[srb]

The SDS series amps also have better cooling through finned heat sinks and have a higher 47K ohm input impedance vs the 7K ohm input impedance of the CDA series, which may provide better matching to some preamps and direct volume controlled source components.
 
Steve

[matt_garman]

Hey matt, I read the christi's reply to your query. But I could not understand it. Could you explain it for the benefit for othe forum members.

Hehe, I'd like to explain it to you, but it's also over my head.    For what it's worth, I did ask Tom about this.  I sent him Cristi's reply for reference as well.  He said he could make some adjustments to the amp that should fix the problem.  He also said he had one other customer using one of Cristi's SMPSes and had a similar issue.

My original intent with the SMPS was twofold: lower power consumption and small physical footprint.  If you read back in this thread, you'll see the first point didn't work out as planned.  The idle power consumption of the amp with the SMPS was actually slightly higher (compared to the linear PS).  To be fair, it was only a watt or three, and my measurements aren't precise by any means (just using a Kill-a-Watt).

Regarding the small footprint goal: you can also see, earlier in this thread, I was having some overheating issues.  Ultimately, I worked with Tom to get a different board.  Also, as per his suggestion, I drilled holes in the bottom of my enclosure.  Understand that the enclosure was just big enough to contain the SMPS plus SDS-470 amp board, and very little extra space.  I ran it for a short while, and didn't have any problems.  But the case was pretty warm to the touch; not enough to cause any audio problems, but enough to make me feel uncomfortable.

I do think my SMPS-powered SDS-470 sounds slightly better than my other SDS470, which is a pre-assembled unit.  But my DIY version also excludes the input gain pots in favor of fixed resistors (not those fancy/expensive Vishay foil resistors, but regular cheap commodity resistors).  I also don't have switchable input, it's XLR only.  So there are more differences than just the power supply.  Now I think I'm going to mod the the linear PS (original from the DIY SDS470 kit) with bigger capacitors and see how that compares to the SMPS.

To help anyone else interested in the Connex SMPS, here's Cristi's reply from the DIY Audio Forums:

Matt: The SMPS800R output voltage will slowly decrease during power off sequence to near zero if the amplifier has an idle current draw of at least 50-100mA. If there is no load or the amplifier consume much less, is possible that the output voltage to fall then when reach about half the power supply to restart switching for brief period, increasing the output voltage with 20-30%. this happens because at power down, the capacitors which store the energy for controller and driver section, will discharge much faster than the large caps, because is much smaller, 100 uF at 25V compared with two 1000uF at 200V, and since the large caps are still charged, the small cap will be recharged in about 2-3 seconds and restart the smps for a brief period till most of the stored energy from the large caps is consumed. On board there is a circuit to prevent the power supply restart, there is a resistor divider which supply one pin of the controller ic with a divided voltage from the mains DC bus, so when this voltage drops below ~240V DC the smps will shut-down and will not restart till the power is applied again. However, to be able to work properly in every corner of the world, where the mains voltage can be substantially lower, I decided to reduce this threshold to about 200V. this means that the smps should work also with 180V AC or 90V AC voltage. If the mains supply voltage is stable, over 200V or 100V then the initial values for the divider can be used, so the smps will not restart once is turned off.
On the other hand, the amplifier itself must feature an UVP and OVP protection, not only for this issue but also to prevent erroneous switching frequencies and damage due to excessive bus pumping when is supplied with too low, and too high voltages. The threshold for UVP must be not lower than 70-75% of the nominal supply voltage and the threshold for OVP must be not higher than 120-125% of the nominal supply voltage for safe and reliable operation.

[matt_garman]

Power supply capacitor upgrade: I'd like to try upgrading the capacitors in the power supply to the 10,000uF.  The Panasonic TSHA series were recommended earlier in this thread, but I can't find a TSHA-series in an 80v value.  What about the T-HA series, such as DigiKey part P10645-ND?

Even those are $12/each.  Six of those would run me $72.  Whereas I can get a whole new linear power supply using six 10k uF 80v caps for $54 from ConnexElectronic.

Any reason to do the capacitor mod myself versus just getting a whole new PS?