[jtwrace]

If anyone gets a 600w version I've posted decent pics in this thread.  I could certianly take more if needed.  I'll will post pics when I have it in my case that will be available to all...

[macrojack]

Is the 600 watt amp the only one that can be run balanced?

[steve k]

Send Tom an email. He's quite helpful.

[Niteshade]

Out of curiosity, is anybody making their own power supplies for their Class D amp modules?

[steve k]

Out of curiosity, is anybody making their own power supplies for their Class D amp modules? Logged

Not that I know of but I wouldn't think it would be difficult. It's a separate board with bridge rectifier and 6 electrolytic caps. Tom provides two different PS boards and a variety of toroids to meet his requirements.

steve

[gitarretyp]

I received the 250 W x 2 @ 4 ohm model yesterday and am pretty impressed with it, sonically.

There are a few issues that have been mentioned in the thread I can hopefully address:

How would these do with a passive pre? Would they be driven well with a standard DAC output level?

I run my system with DAC direct to amp, so I have some experience here. This setup works with a few caveats: The amp has a very low input impedance (something I asked Tom to which he never responded). I estimate it at ~ 3 k ohms. This means that your dac needs to have output caps ~ 6 uF or larger (6 uF puts the high pass at ~ 10 Hz and should have minimal effect on most music). Also, the amp's gain is a few dB lower than the spec Tom gave me (he said 34 dB, I measure closer to 31 dB). 31 dB is a little low for my tastes in a passive system. The gain is adjustable by changing resistors in the feedback circuit. Tom speced the 120 x 2 amp at 26 dB of gain in its stock configuration, which is too low for most passive systems. He can adjust the gain for you, but be sure to ask before you order.

I've seen much discussion on swapping the caps on the output filters. A few notes on this: This amp and the 120 x 2 are based on the IRS2092 chip and is pretty similar to this amp design. The caps being discussed are in the low pass filter and the zobel network --both caps are parallel to the loudspeaker. Since these are on the output of the amp, some special considerations need to be taken into account because you're dealing with RF rather than normal audio frequencies (see this article for a nice primer on Class D and its peculiarities). The ideal caps for this position are low inductance (I've never seen an audiophile cap's inductance measured) with leads as short as possible. Low inductance is importance because at RF frequencies, caps act like inductors and inductors act like caps. So, the low pass filter become a high pass filter at RF frequencies if the parts aren't carefully chosen. Long leads hanging off the board are bad because the caps effectively become small FM antennae. Because of these issues, the best caps for these positions are radial polyester film caps.

Another note on the output filter cap: looking at the schematic, the output filter seems to be optimized for 4 ohm loads and is a bit too high Q for 8 ohm loads, leading to a rising response above ~ 5 kHz --verified by my own measurements. So, tweaking the value of the 0.47 uF cap might be in order.

[poseidonsvoice]

gitarretyp,

Thanks for the input. That input impedance is about the lowest I have every seen in an amplifier. Indeed, it would be important to make sure the output impedance of your preamp/dac isn't much higher than 300 ohms to work well.

The output filter cap is not something I was planning on fooling around with, as I was going to purchase a pair of the amp boards to build a sub amplifier, i.e. 500 watts/8 ohms and about 750 watts into 4 ohms (when each board is bridged).

Anand.

[mr_bill]

Hi,
Knowing that the input impedance is very low on these amps, would a preamp or dac with an output impedance of 100 ohms work and not create a mismatch or loss of frequency extremes?

[jtwrace]

Hi,
Knowing that the input impedance is very low on these amps, would a preamp or dac with an output impedance of 100 ohms work and not create a mismatch or loss of frequency extremes?

My tube buffer (Dodd) has an output impedance of 130 Ohms.  No problem at all.

[steve k]

My VTL5.5 has an output impedance of 200 ohms and they're a perfect match.

[gitarretyp]

Anything with an output impedance less than 300 ohms is certainly fine from an impedance matching standpoint. I would be more concerned with the size of the output coupling caps installed on most dacs/preamps producing a high pass filter in the audio band.

[steve k]

I would be more concerned with the size of the output coupling caps installed on most dacs/preamps producing a high pass filter in the audio band.

Hmm. I'm not sure what you're getting out there. FWIW, I think I have better overall extension at both ends of the spectrum with the Class D amp than I had with my tube amps. I suppose that's a question for Tom. As far as impedance matching, my friend Mark uses a Conrad Johnson preamp with his Class D. The Connie has an output impedance of 800 ohms (above the 600 ohms recommended by Tom) and has no issues.
steve

[gitarretyp]

Hmm. I'm not sure what you're getting out there. FWIW, I think I have better overall extension at both ends of the spectrum with the Class D amp than I had with my tube amps. I suppose that's a question for Tom. As far as impedance matching, my friend Mark uses a Conrad Johnson preamp with his Class D. The Connie has an output impedance of 800 ohms (above the 600 ohms recommended by Tom) and has no issues.
steve

Here's a measurement of the amp driving my speakers (measure at the amp's output)

(ignore the red and grey lines)

The green line is with ~ 6 uF caps on my DAC's output and the yellow line is ~ 3 uF. You might not notice the difference if your speakers don't have output below 40 Hz.

[steve k]

Interesting. I'm biamping my speakers with the Class D on top so it could be that I haven't noticed any bass rolloff. I will know soon though as I'm going to try the 250wpc/8 ohms amp on the bass. My speakers go down to 32Hz +/-3dB. Has anyone else running their amps full range noticed a bass rolloff?
steve

[6SN7]

I have Magnepan MG12s and whether I use my sub or not, bass with the class Ds (full range mono blocs) goes deeper and is tighter than with my tube mono blocs.
Bob

[pjchappy]

(Edit. . .ooops. . .misread your post regarding the caps and a DAC. . .so, stupid question  ) 

The roll-off isn't significant--looks to be -3 dB @ 20 Hz worst case and -1 dB @ 20 Hz best case.  However, would this concern those who do plan on using these for sub activities?  Of course, EQ is a possible solution for those frequencies, after any room gain.

Again, I'm truly asking out of ignorance.  Also, the high-end starts jumping up pretty good after 10 KHz.  That can also be tamed with EQ (for those that go that route), but wouldn't this concern others?  Regardless, it sounds like people love these damn things!   

Simply based on this thread, these amps have jumped to the top of my list for my future speaker purchase in a few months or so.  I plan on going active with them and depending on configuration, will need at least 4, but possibly 6, channels of amplification. . .7, if I decide to replace the amp I currently have for my sub.

Thanks,


Paul

Here's a measurement of the amp driving my speakers (measure at the amp's output)

(ignore the red and grey lines)

The green line is with ~ 3 uF caps on my DAC's output and the yellow line is ~ 6 uF. You might not notice the difference if your speakers don't have output below 40 Hz.

[poseidonsvoice]

Here's a measurement of the amp driving my speakers (measure at the amp's output)

(ignore the red and grey lines)

The green line is with ~ 3 uF caps on my DAC's output and the yellow line is ~ 6 uF. You might not notice the difference if your speakers don't have output below 40 Hz.

Jason,

Did you mean the green line is for 6 uf and the yellow line is for 3 uf? The green line seems to be more extended in the low end to me  Given the minimal differences at 20Hz Paul, it wouldn't deter me from using this amp for sub bass duties.

And what is the dB level at 20kHz? Is that really +4dB? If so, tweaking that 0.47uf cap may be useful, let us know what you settle on.

Anand.

[gitarretyp]

Sorry, I switched the 6 and 3 (now corrected). 3 uF results in a first order high pass at ~ 18 Hz, and 6 uF is half that.

Yes, that really is +4 dB at 20 kHz. It's audible but mostly adds "air/spaciousness" and shimmer to cymbals that probably shouldn't be there, i.e., it's noticeable but not in a bad way. If you take a look at page 11 of this data sheet, you'll see that into an 8 ohm load the amp is ~ +2 dB at 20 k. I think the Nathan's are probably closer to 16 ohm at 20 k; thus, the more boosted top end.

[poseidonsvoice]

Jason,

Do an impedance plot on your Nathan's to be sure. The Abbey's are dead flat from 2Khz up to 20khz at about 10-12 ohms to be honest. Given that they both have the same tweeter and stem from the same philosophy, I wouldn't think the Nathan's should be any different. Still, the top end will be a little boosted.

Anand.

[shadowlight]

Can someone take additional pictures and post please.  I received the transformer today and started to put it together and ran into a snag right away.

Here is what I have done so far:

1) Took the center blue/green cables and connected it up to CT (on the powersupply board.
2) Connected the outside blue wire to AC-N
3) Connected the outside green wire to AC-L

Now the big question.  There are two brown wires coming out of the transformer and I do not see any documentation on where to connect it up.  Same question related to the two red and black wires.

Close up pictures of the amp to powersupply connections would be much apprericiated.

TIA,
Deepak