Removing Limiter From An Amp

[tomtoo]

Even though I can see a couple of reasons to NOT use it as a subwoofer amp The above is actually not bad advise. (So I modified my previous post slightly) Depending on your use case the flaws might not be a total showstopper, but for serious HT use its going to suck real bad

Now that you have it, at least try it. You might like the punchy sound from the additional distortion, 50Hz peaking and rolled of last octave
On the bright side it does have an (although non defeatable and maybe not 100% intentional) Infrasonic filter which might work well with some ported subs with limited excursion capability .

Good to use with vinyl.

 

[tomtoo]

This amp is rated at 450W/8 ohm.

I see no reason why this amp would not be suited for low frequencies.
It is possible that besides the temperature protection, over current protection and short circuit protection there is a 'tightly tuned' DC protection circuit that will kick in at constant very low frequencies at high amplitude.
There will not be a limiter and the amp will not have reduced power at 20Hz.

I see no reason why it could not be used as a sub amp.

Measured at 220w and already 5dB down at 20Hz. This thing measures strange for a amp in 2022.

 

[Trdat]

This amp is rated at 450W/8 ohm.

I see no reason why this amp would not be suited for low frequencies.
It is possible that besides the temperature protection, over current protection and short circuit protection there is a 'tightly tuned' DC protection circuit that will kick in at constant very low frequencies at high amplitude.
There will not be a limiter and the amp will not have reduced power at 20Hz.

I see no reason why it could not be used as a sub amp.

Do you still stick by this comment after reading the review on Soundtown? I just don't know if the comment above was before or after knowing there is a review on the amp.

https://www.audiosciencereview.com/...sound-town-st-updm4c-4-channel-pro-amp.29033/

 

[Trdat]

Let me add maybe a german point of view.

First you need someone that has realy the expertise. They not work for a apple and a egg. So finding the problem(lets imagine it would be the PSU), changing it is easy 4h work. What is in germany easy 400Euro without parts.

Labour is much cheaper in my country but in saying that, not sure if I would want my technician to go through such a process if its back breaking and I can potentially buy a better amp than I get your point. Thanks for your German point of view really puts it in perspective.

 

[Mnyb]

"Limitations" (probably a limit to the design itself) are there for a reason , it will probably go very bad if you managed to work around this "protection" or whatever it is.

It's a complete circuit . if it where some magical limit that could be removed it would not be there in the first place and the amp would measure better to begin with ?

 

[tomtoo]

"Limitations" (probably a limit to the design itself) are there for a reason , it will probably go very bad if you managed to work around this "protection" or whatever it is.

It's a complete circuit . if it where some magical limit that could be removed it would not be there in the first place and the amp would measure better to begin with ?

Its a "pro" amp. Under normal circumstances nobody cares about 20Hz in a small PA system. If a PA goes down to 40Hz its a high end system. There is not much down there from a music point of view, and from a technical point of view there starts the hell if not more early. Dont know how deep dave rats monster subs go, but for things like this you need a nuclear power station near by. And amps that can easy drive some toasters. Maybe they just did not care about. If this thing works nominal at 40Hz all is good for the planed usage.

 

[solderdude]

Do you still stick by this comment after reading the review on Soundtown? I just don't know if the comment above was before or after knowing there is a review on the amp.

https://www.audiosciencereview.com/...sound-town-st-updm4c-4-channel-pro-amp.29033/

Not intended for Hifi but rather for sound reinforcement at a low price.
I would not bet my life on it these are the same amps, the rear and front panel have a very different layout.

I would not buy it but if I had one and needed an amp for a sub I would hook it up and hear what it does.
Also would not use an amp with (small diameter) fans for cooling in any case.
This 'belongs' in a 19" rack for low budget PA work on-site.

 

[tomtoo]

There iam on @solderdude side.

Hook it up. If it not performs well for you, sell it.

 

[JeffS7444]

The easiest way for a manufacturer to limit low frequency response is to use an undersized input coupling capacitor. Besides acting as a DC-blocker, said capacitor in combination with Miller capacitance of input circuitry can act as a high-pass filter. There's a simple formula for calculating the ideal capacitance to use for a desired roll off point, but at the moment, I can't find it. Worth the effort? I don't know.

 

[mhardy6647]

The easiest way for a manufacturer to limit low frequency response is to use an undersized input coupling capacitor. Besides acting as a DC-blocker, said capacitor in combination with Miller capacitance of input circuitry can act as a high-pass filter. There's a simple formula for calculating the ideal capacitance to use for a desired roll off point, but at the moment, I can't find it. Worth the effort? I don't know.

Do you mean (?)

where f(sub)c is the -3 dB point.

 

[solderdude]

Clearly the LF roll-off is not 6dB/octave so there seems to be some steep filtering going on here.
The extra problem with the SoundTown is the DC protection and, very likely, the steep HPF filter is essential as to not shut the amp down when loud bass notes are played.
So changing the roll-off means changing the entire filter tuning and is not a matter of simply increasing 1 or 2 coupling caps in value.

 

[DVDdoug]

This something very strange and I can't think of any reason for including intentionally. 20Hz doesn't put any more stress on the amplifier than any other frequency.

DC protection is a likely possibility and that could probably be removed/bypassed. DC is zero Hz so maybe it's not waiting long enough for an AC polarity change and it "thinks" it's putting-out high-power DC.

But I can think of a couple of other design defects that might cause something like this. And class-D amplifiers are difficult to design and all kinds of strange things can happen.

There are (sometimes) reasons for a regular high-pass filter but a regular (linear) filter would limit frequency response at all power levels... You don't want to waste energy on sub-sonics that you can't hear or low frequencies that your speaker can't reproduce. Not only does it unnecessarily stress the amplifier and speaker, it reduces the dynamic range of what you can hear. Or, you can end-up distorting the part that your speaker and produce and that you can hear.

 

[JeffS7444]

Maybe there's digital filtering going on?

@tomtoo may have intended it as a tongue-in-cheek remark, but that low-frequency cutoff really does look perfect for use with vinyl.