Jeff Rowland

I'm not familiar with the specific amp, but in general I would not expect a bridged amp to provide good results when driving your particular speakers. As can be seen in the impedance curve shown here, the impedance of your speakers is in the vicinity of 3.5 to 4 ohms between 50 and 100 Hz, where lots of energy is often required, and is close to 3 ohms around 25 Hz. In bridged mode the amplifier will "see" a load impedance equal to half those values, i.e. 1.5 to 2 ohms, and of course most amps will not perform at their best when dealing with such low impedances.

Regards,
-- Al

Hi Dan (Islandmandan),

I found this impedance curve for the HPD 315he, which confirms (as might be expected for a Tannoy) that its impedance characteristics are very benign. (If the link just opens at flickr.com instead of opening at the specific photo, paste the url https://www.flickr.com/photos/89700142@N03/8344329373 into the address bar of your browser). Note, btw, that in contrast to most such curves that are presented this one extends down to 1 Hz. So the somewhat low impedance magnitudes at the left end of the curve essentially represent DC resistance, and are reasonable values that can be expected to be inconsequential.

I also looked at the web pages on the amp module and the kit, and the manual.

The bottom line is that I don't doubt that the amp can drive the speaker in a reasonable manner in bridged mode, but of course how the resulting sonics would compare with stereo mode is speculative.

Another possible concern, though, is that I note that the amp module's input impedance is spec'd at a low 7K, and I suspect that in bridged mode it is likely to be around half that amount, or only 3.5K. That will be non-optimal for many preamps, especially the many tube-based preamps which employ coupling capacitors at their outputs, the capacitors typically resulting in substantial output impedance rises at deep bass frequencies.

But if you don't specifically envision an impedance incompatibility with the preamps you may use, given the very low price of this kit it would seem to be a reasonable risk. Also, perhaps it might be helpful in reaching a decision to try providing the kit you already have with a mono signal and running it in bridged mode with one speaker.

Best regards,
-- Al

P.S: Kijanki, thanks for both the input and the compliment :-)

Ricred, what can often be a very major contributor to lack of bass impact is the acoustic effect of the rear wall (the wall behind the listening position). I can't tell from your system description photos what that distance is, but under typical indoor environmental conditions rear wall reflections will produce a suckout (i.e., a lack of bass) centered at a frequency in Hz of about 282 divided by the distance in feet between the listener's head and that wall.

So if you were say five feet from that wall, there would be a suckout centered at about 282/5 = 56 Hz. In my experience that effect can often be MUCH more significant than differences in bass performance between most electronic components.

Good luck as you proceed. Regards,
-- Al

03-09-15: Ricred1
Speakers are as follows:
the rear of the speakers are 4' 2" from the front wall
8' apart, 4' 8" from sidewalls, I sit 10'away, my head is 6'away from the rear wall
I have 2 GIK Soffit Bass traps in each front corner from floor to ceiling, 6 GIK 242 panels-2 each on the left and right at the first reflection points and 2 on the ceiling

OK, so rear wall reflections will result in a suckout centered at a frequency of 282/6 = 47 Hz. It's hard to say how far above and below that frequency the suckout would extend to an objectionable degree, but I wouldn't be surprised if the affected frequency range is wide enough to be a significant contributor to the issue.

For experimental purposes, just to gauge the significance of that effect, if practical you might temporily relocate the two bass traps that are in the front corners to a position such that both of them are in a line extending directly back from the listening position. Perhaps one immediately behind the listening position, and one up against the wall. And it would probably be most useful to compare sonics between having the traps in that position and having them out of the room altogether, so that the comparison wouldn't involve changing two things at once (i.e., moving the traps away from the front corners, and placing them behind the listening position).

Also, again for experimental purposes, you might try to determine how severe that effect seems to be at differing listening distances. In doing so, it might be helpful to purchase a test CD, such as this one, further described here. Track 17 would facilitate assessment of frequency response flatness in the bass region.

Bombaywalla, I agree with your comment. It would be unusual for an amp to be able to fully quadruple its max power rating into 8 ohms when bridged, much less into 4 ohms, due to the current and thermal limitations you referred to. Although for an 8 ohm load the OP's 525 comes surprisingly close to doing that, the 8 ohm ratings being 250 watts and 950 watts for stereo and bridged modes respectively.

Best regards,
-- Al

The output of the DirectStream DAC is transformer coupled, and in that sense can loosely be considered to be passive, but unlike resistive-based passive preamps it has an output impedance that is low enough to be comparable to that of most active preamps, both tube and solid state. That is confirmed in John Atkinson's measurements.

Given that, as well as the 40K balanced input impedance of your amp, as well as PS Audio's claim that its volume control mechanism does not lose resolution at any setting (which if it occurred at all would occur at low settings of the control, not high settings), I see no technical reason that would call for the insertion of a preamp. Which is not to rule out the possibility that you might find adding a preamp to be subjectively preferable. But FWIW my own bias is to put the burden of proof on adding anything to the signal path that is not clearly necessary, and in this case I don't see it as being clearly or technically necessary.

The reason for the relatively high settings of the volume control you are using are most likely the combination of the "full scale" (maximum) output voltage of the DAC being a bit lower than usual, and the sensitivity of your speakers being somewhat low (Stereophile measured the speakers as 86.6 db/2.83 volts/1 meter, which for their 4 ohm nominal impedance is 83.6 db/1 watt/1 meter).

So as long as you never find yourself wanting to turn the volume control up beyond the top of its range, I wouldn't attach any significance to the fact that you are using it in the upper part of its range.

A couple of things to check, though:

1)Apparently the DAC's output can be set to two different levels via the menus. Check to see that it is set for the high level, i.e., that the output attenuator that is provided is deselected.

2)Check that the unit has the latest firmware (version 1.2.1) installed. User comments as well as PS Audio indicate that that update tightens up and improves the bass. I would expect that you can determine the firmware version somewhere in the menus, or perhaps it is displayed briefly at startup. If it is an earlier version, you can easily download and install that version per the instructions at PS Audio's site.

Regarding the bass traps, it's of course sometimes possible to have too much absorption in a room as well as too little absorption, especially from a subjective standpoint, and depending on the speakers, their placement, etc. I suspect that the main reason for the more powerful bass reported in your latest posts was removal of the traps from the front corners, rather than their placement behind the listening position, in part because the change was perceivable at other than the listening position. Hopefully after doing the remaining experiment (removing the traps from the room altogether), and some further listening, you'll be able to determine which of the four possibilities is preferable (traps or no traps in front; traps or no traps in the rear).

Continued good luck. Regards,
-- Al

Thanks, Bombaywalla. I see what you're saying. The module datasheet shows that when it is operated with 120 VAC it is rated to deliver 600 watts into 2.7 ohms in stereo mode, although with just one channel driven. That corresponding to an output current of 14.9 amps.

While the max rating in bridged mode that you cited of 700W into 4 and 8 ohms corresponds, for 4 ohms, to 13.2 amps from each channel, simultaneously but with the channels being operated out of phase.

A couple of inconsistencies in the numbers, though, which I don't know how to reconcile:

1)The 950 watt figure I cited for bridged mode, for an 8 ohm load, comes from the Rowland site (click "specifications"). That number being even higher than the 900W number that is indicated in the module datasheet for 230 VAC operation, for an 8 ohm load in bridged mode.

2)I would think that the indication in the module data sheet of 30 amps peak output current is likely to refer to the peak of a sinusoidal waveform. (For example, I've seen credible indications that Pass Labs specifies peak output currents on that basis). While the currents that can be extrapolated from the power ratings based on Isquared x R would be rms values. For a sine wave, 30 amps peak of course corresponds to 30 x 0.707 = 21.2 amps.

As I say, I'm not sure how to reconcile all of that. Assuming that the module for which Kijanki provided the datasheet is truly the one used in the 525, I suppose it all adds up to further confirmation of how, as he indicated in his last post, power specs are often not defined on a consistent basis.

Best regards,
-- Al

03-10-15: Bombaywalla
Then, in bridged mode if each amp in the stereo module is outputting 13.2Arms that total is 26.4Arms which is greater than the 21.1Arms that the integrated SMPS is capable of supplying!! So, is the amp really able to output 700W into 4 or 8 Ohms in bridged mode (as in the spec sheet)?

I'm not 100% certain of this, but I believe the way to look at it is that since in bridged mode the two channels are operated out of phase with each other, and the load is connected between the two + output terminals of the amp, from the perspective of the power supply it is supplying 13.2 amps, not 26.4 amps, when supplying max rated power into 4 ohms in bridged mode.

In other words, the 13.2 amps that is flowing through one channel does not add to the 13.2 amps that is flowing through the other channel, because it is the same 13.2 amps!

While if both channels are driven in stereo mode the power supply would of course have to supply enough current to support the total amount of power being drawn from both channels. Note, though, that all of the output power ratings indicated in the module datasheet for stereo mode are based on only one channel being driven.

... all these numbers don't jive with each other further adding to the confusion of what the amp is really capable of power output-wise. in such a case, I'd be inclined to take a slightly conservative path & side with the class-D amp module manuf...

Agreed (except that strictly speaking I believe it would be "jibe," not "jive" :-)). Perhaps all that can be said with certainty is that this is a very powerful amp, but one which should not be expected to perform well when used in bridged mode with speakers having impedances which drop below 4 ohms at frequencies for which lots of power is likely to be required.

Best regards,
-- Al

Glad I was able to help, Ricred, and thanks for the very nice acknowledgment. Enjoy!

Best regards,
-- Al

I'm uncertain as to how well connecting a sub to the DAC's RCA outputs would work. It would certainly function. But without clear information as to the design of the DAC's output circuits I would have some concern that doing so might have at least slight adverse effects on the signals provided to the Rowland amp, and thus to the main speakers, unless the circuits driving the two outputs are totally independent. That concern being heightened by the fact that as with the line-level inputs of most subs the input impedance of the F112's unbalanced inputs is low, in this case only 10K.

And I note that in the entire DAC manual the only statement that is bold-faced is "We do not recommend using both outputs at the same time."

And then of course there is the question of how good a match there would be between the sonics of the sub and the main speakers, about which I'm not in a position to comment.

Also, of course, there are other subs which provide speaker-level inputs, such as the RELs. However, the 525 amp is described as having "balanced topology implemented throughout all power and input/output circuits," which leads me to suspect that it may not be ok to connect the ground wire from such a sub to the amp's negative output terminal. And the amp does not appear to provide any other suitable ground connection point. In which case connecting that wire to a chassis screw on the amp would **probably** work ok, but again there is uncertainty.

Best regards,
-- Al