UCD180 questions

[Bruno Putzeys]

Damping factor is a highly overrated spec. The damping of a loudspeaker's mechanical resonance is largely effected by the back emf causing dissipation in the voice coil resistance. For the Q of this system to be as intended, the residual series resistance (amplifier and cables) should be low compared to that of the voice coil.

This is one reason why we want to have a low output impedance on the amplifier and why it is sometimes referred to as damping factor.

Now, whether this residual resistance is one tenth or one thousandth of the voice coil resistance has very little effect on the final Q.

There seems to be an incorrect intuition among many audio folks that a source with a very low output impedance will have superlative control over the speaker cone's movements. In fact, even if you put a dead short across the speaker's imput terminals, you can still move the cone quite easily. There is a limit to the amount of control the source can have and that limit is... the voice coil's resistance!
Actually this is a good thing, otherwise it would be completely impossible to tune the loudspeaker for a flat response. Closed speakers would roll off at relatively high frequencies and ported ones would have an undamped resonance at the port frequency.

What does this mean for the concept of "damping factor"? That the formula DF=Znom/Zout is completely meaningless.
A meaningful formula would be DF=Rdc/(Zout+Rdc). This formula correctly reflects that the damping factor approaches, but never exceeds unity as amplifier output impedance drops.

So why then do amps with ultralow impedance sound so well-controlled in the low-end? Simple coincidence. Such amplifiers have very high DC loop gains causing their distortion to increase with frequency. The colouration at high frequencies fools the ear to believe that the bass is tight - tighter than is physically possible. Bass control freaks are actually listening to a form of euphonic defect.

[Bruno Putzeys]

Originally posted by maxlorenz

I recently finished a transformer based passive volume control (AKA TVC) wich is wonderful, at least to me.

1) Are balanced and unbalanced input impedances different?

2) Are there versions with higher input impedance to mate better with passive pre's? Or, how can I increase input impedance, without compromising sound?

3) Will balanced input sound better than unbal?

4) Is it wise to separate the PSU box from the power amp box?

5) Is it comparable if I use one 220VAc to 2x32VAC transformer with separate rectifiers/caps instead of two completely separated PSU's, in terms of noise and channel crosstalk, etc..?

1) Both input lines are tied to ground with 10k.
2) If you want higher input impedance, an op amp other than the NE5532 should be used (e.g. OPA2134 or AD8620). Then the input "termination" (bad word) can be upped to 100k or higher if you want.
The 5532 has rather high input bias current, and 10k is installed to counter this.
3) The modules' inputs are differential. They do not require to be driven by opposite-phase signals. The most important thing to do is to use the differential inputs to extract the output signal from your preamp referencing the preamp's ground. Outside audio this is known as Kelvin sensing.
There is some discussion on this thread explaining how to best connect an unbalanced output to a differential input.
4) No.
5) Either use two transformers with each their own rectifiers and storage (dual mono) or one transformer with only one rectifier/storage. Trying to use one transformer with two sets of rectifier/storage will invite some very hideous hum.

[loudandclear]

Bruno,

Besidws changing the Opamp, what else is required to increase the input impedence?

Is is a simple resistor change?

Thanks

[pburke]

Originally posted by Bruno Putzeys
Damping factor is a highly overrated spec. The damping of a loudspeaker's mechanical resonance is largely effected by the back emf causing dissipation in the voice coil resistance. For the Q of this system to be as intended, the residual series resistance (amplifier and cables) should be low compared to that of the voice coil.

just a side note - I have no "cone speaker" nor a speaker cabinet. There's quite some "voice coil" to control, about 5 square feet of magnets, which it drop to 2.5ohms somehwere around 100hzl. Magnepan speakers eat amps for breakfast. I know of people who finally found what they have been looking for using 2x700w/channel to drive them. If I get anywhere into that ballpark, I should be very happy with the UCD amps as well.

What you describe about the HF distortion of low impedance amps - I could theoretically live with that in my planned biamp setup where the amp I am mostly concerned about will be driving the bass panel and not see more than 250hz (and rolling off at 18db/oct above that).

From what I have seen so far about the modules, I think the UCD400's will just fine with my speakers. What size of transformer would you suggest for a 2-channel amp to drive just the bass panels up to 250hz with two UCD400s. This is a 2.5-4 ohm load and I would like to have a good margin of needless extra current capacity - I guess 1000 or 1200va would do, or did I misinterpret one of your earlier posts about what it takes to significantly increase the 8-ohm rated power at half the load?

Sorry if my questions sound silly at times - this will be my first power amp project. I've spent most of my DIY time with DACs and preamps so far. Also - I do have great amps - I just want even better amps at half the price. I want to learn as much as possible before I begin in order to do this right the first time

Peter

[Bruno Putzeys]

Originally posted by loudandclear

Besidws changing the Opamp, what else is required to increase the input impedence?

Is is a simple resistor change?

First you change the op amp, then the two resistors that are nearest the audio input connector (10k to ground).

[Bruno Putzeys]

Originally posted by pburke
What size of transformer would you suggest for a 2-channel amp to drive just the bass panels up to 250hz with two UCD400s. This is a 2.5-4 ohm load and I would like to have a good margin of needless extra current capacity - I guess 1000 or 1200va would do, or did I misinterpret one of your earlier posts about what it takes to significantly increase the 8-ohm rated power at half the load?

Sorry if my questions sound silly at times - this will be my first power amp project. I've spent most of my DIY time with DACs and preamps so far. Also - I do have great amps - I just want even better amps at half the price. I want to learn as much as possible before I begin in order to do this right the first time

800VA would already be sufficient for what you are planning. In "consumer audio" we'd be using a 500VA transformer to power 800W worth of amplifier, so 800VA would already provide the margin you're looking for.

Keep in mind that class D amplifiers always deliver higher output current than the power supply current they're drawing. (Pin=Pout but Uin>Uout)

[Jan-Peter]

To all Customers who bought a UcD180,

I have some importend points;
First of all I recommend when you like to use an other op amp to watch the maximum voltage of this device for instance by the AD8620 you have to reduce the voltage from 15V to 12V. However we don't us a voltage regulator (in an earlier post I made this mistake) but a transistor with a zenerdiode, so you have to replace this SMD zenerdiode. For details which diode must be replaced please send me an email.

Second, if you like increase the inputresistance you can go to 100K but because we use a NE5532 in the frontend you can not remove the coupling caps!!! For that you have to replace to a lower offset op amp as an OPA2134 or an AD8620 (change also the zenerdiodes from 15V to 12V)

We are testing the AD8620, when this is a major improvement we will offer a UcD180 version with the AD8620.

For some detailed questions please mail me direct.

Best regards,

Jan-Peter

www.hypex.nl

[wytco0]

Jan-Peter, thanks for the updates, have you considered putting some of this info on your web site along with some of the other interesting stuff that's been written here? I know you have a link to this thread but its now very long and it becomes difficult to find info is such long threads.

Things that I think would be useful :-

Description of power supplies recommended, possibly some actual designs, eg dual mono, single; tranformer ratings etc

Description of signal connection, balanced vs. unbalanced.

Protection information, Switch on etc.

Cooling recommendations.

Opamp discussion

Info re cooling requirements, eg making it clear that although they run cool some heatsinking is needed.



Regards
TC owner of module 70 and 71 !

[Jan-Peter]

Wytco0,

We are working on a paper where all this info is written. But the info on the Forum goes fasters as we can write it on paper

At the moment I am doing a temperature test on the UcD400.
The UcD400 is mounted on an aluminum plate of 190x220mm in free air.
Dummyload is 4 Ohm.
UcD400 delivers 150W.
Total eff. is 91.5%.

The whole setup is now already playing for 1,5 hour and the temperature next to the fets is 61oC.

Regards,

Jan-Peter

www.hypex.nl

[classd4sure]

Hi,

Would noise suffer at all with 100k input impedance?