Questions About the Event TR5 Schematic

Paul wrote:
>
> Phil Allison wrote:
>
> >
> > Similar S-K filter stages are connected around the two LM3886 power amps - after scaling each output (eg R7 & R8) to give unity gain. This doubles the roll off slopes to 24db/octave.
> >
>
>
> If the roll-off slope for the TR8 is -24dB/octave, and there is
> -6dB/octave per element, then it looks like this can be considered a 4th
> order filter, right? Where did you get the -24dB/octave?
>


** Look at the TR8 schem carefully - the LM3886s have additional feedback paths around them aside from the usual gain setting networks.

U1 has a filter path with the same component values used for HF crossover U3A while U4 has one the same as LF crossover U3B.

As the filters are in series, the ultimate roll of slope is double that of each - or 24dB / oct.


..... Phil

On 10/29/2018 12:56 AM, Phil Allison wrote:
> Paul wrote:
>>
>> Phil Allison wrote:
>>
>>>
>>> Similar S-K filter stages are connected around the two LM3886 power amps - after scaling each output (eg R7 & R8) to give unity gain. This doubles the roll off slopes to 24db/octave.
>>>
>>
>>
>> If the roll-off slope for the TR8 is -24dB/octave, and there is
>> -6dB/octave per element, then it looks like this can be considered a 4th
>> order filter, right? Where did you get the -24dB/octave?
>>
>
>
> ** Look at the TR8 schem carefully - the LM3886s have additional feedback paths around them aside from the usual gain setting networks.
>
> U1 has a filter path with the same component values used for HF crossover U3A while U4 has one the same as LF crossover U3B.
>
> As the filters are in series, the ultimate roll of slope is double that of each - or 24dB / oct.
>

I see. I checked the roll-off of the HF crossover U3A,
and indeed it is about -12dB/oct, or -40dB/decade. The cutoff
frequency is about 2,729 Hz, which matches roughly what I heard
on the bench.

So crossover U3A is a 2nd order filter.

Can you safely assume the order of this type of active filter will
be the same as the number of reactive elements (capacitors, or
inductors), which in this case is two caps?

Just like you can simply count reactive elements in a totally
passive filter, to get the order of the filter?

Paul wrote:
>
>Phil Allison wrote:
>
> >
> > ** Look at the TR8 schem carefully - the LM3886s have additional feedback paths around them aside from the usual gain setting networks.
> >
> > U1 has a filter path with the same component values used for HF crossover U3A while U4 has one the same as LF crossover U3B.
> >
> > As the filters are in series, the ultimate roll of slope is double that of each - or 24dB / oct.
> >
>
> I see. I checked the roll-off of the HF crossover U3A,
> and indeed it is about -12dB/oct, or -40dB/decade. The cutoff
> frequency is about 2,729 Hz, which matches roughly what I heard
> on the bench.
>
> So crossover U3A is a 2nd order filter.
>
> Can you safely assume the order of this type of active filter will
> be the same as the number of reactive elements (capacitors, or
> inductors), which in this case is two caps?
>
> Just like you can simply count reactive elements in a totally
> passive filter, to get the order of the filter?
>

** Normally yes.

If the changing impedance of a cap with frequency is reflected in the stage's output, then it is likely adding 6dB / oct to the final slope.

18db/oct active stages normally have 3 caps.



..... Phil

On 10/29/2018 11:19 PM, Phil Allison wrote:
> Paul wrote:
>>
>> Phil Allison wrote:
>>
>>>
>>> ** Look at the TR8 schem carefully - the LM3886s have additional feedback paths around them aside from the usual gain setting networks.
>>>
>>> U1 has a filter path with the same component values used for HF crossover U3A while U4 has one the same as LF crossover U3B.
>>>
>>> As the filters are in series, the ultimate roll of slope is double that of each - or 24dB / oct.
>>>
>>
>> I see. I checked the roll-off of the HF crossover U3A,
>> and indeed it is about -12dB/oct, or -40dB/decade. The cutoff
>> frequency is about 2,729 Hz, which matches roughly what I heard
>> on the bench.
>>
>> So crossover U3A is a 2nd order filter.
>>
>> Can you safely assume the order of this type of active filter will
>> be the same as the number of reactive elements (capacitors, or
>> inductors), which in this case is two caps?
>>
>> Just like you can simply count reactive elements in a totally
>> passive filter, to get the order of the filter?
>>
>
> ** Normally yes.
>
> If the changing impedance of a cap with frequency is reflected in the stage's output, then it is likely adding 6dB / oct to the final slope.
>
> 18db/oct active stages normally have 3 caps.
>

Ok, excellent. There is order in the universe!

Thanks for sharing your knowledge, Phil, it's been a great help!

Hi Everyone, sorry to wake this thread up, but I recently acquired a set of TR5's and it seems that one of them has probably a bad input sensitivity pot - very low volume and it feels like the wiper isn't making contact whatsoever which seems to be pretty common. On the working one, I can feel the drag of the wiper, but this one feels open. I'd like to replace them both, but having a hell of a time sourcing a drop-in replacement. They're 2k it looks like, but small, rectangular and it looks like the long shaft to clear the back panel is integrated. I can't find anything similar anywhere and don't really want to mod anything at all if I don't have to, so just wondering if anyone knows off the top of their heads where to get them, or at least a specific PN (it looks like it has "DI YA B2K" and 0406 on it.) I've seen where people have replaced them out there, but of course no mention of what they replaced them with (Grrr.)

Thanks, and again sorry to jump on this, but you guys sound like you may be my last and only hope... =]

Terry
(Some guy in Iowa)

Originally Posted by TheIronChicken

Hi Everyone, sorry to wake this thread up, but I recently acquired a set of TR5's and it seems that one of them has probably a bad input sensitivity pot - very low volume and it feels like the wiper isn't making contact whatsoever which seems to be pretty common. On the working one, I can feel the drag of the wiper, but this one feels open. I'd like to replace them both, but having a hell of a time sourcing a drop-in replacement. They're 2k it looks like, but small, rectangular and it looks like the long shaft to clear the back panel is integrated. I can't find anything similar anywhere and don't really want to mod anything at all if I don't have to, so just wondering if anyone knows off the top of their heads where to get them, or at least a specific PN (it looks like it has "DI YA B2K" and 0406 on it.) I've seen where people have replaced them out there, but of course no mention of what they replaced them with (Grrr.)

Thanks, and again sorry to jump on this, but you guys sound like you may be my last and only hope... =]

Terry
(Some guy in Iowa)

Never mind I think - I'm going to try these: https://www.ebay.com/itm/1PC-2090F-2...53.m2749.l2648
and cut the old plastic shaft off and JB-Weld it to the new one or something.