Hi,
if you implement a high pass filter between two components then the more predictable location is at the input of the receiver.
As you have noticed locating the High Pass at the output of the transmitter requires the builder/manufacturer to guess at what input impedances are likely to be fitted next in the sequence.
For this reason some transmitters get fitted with high value electrolytics to allow bass to get through to all reasonable receiver impedances. I have seen 220uF Non-Polar electrolytics used for just this eventuality.
You as builder can decide that all your receiver equipment will have an input impedance that is compatible with all your transmitting equipment. Similarly you can choose the transmitters' DC blocking cap values to be compatible with all your receiving equipment. You can also decide what quality of DC blocking cap to use or even whether it is needed. All the choices are your's.
If I were being asked for typical values, I would tend towards 10uF polypropylene film & metal foil on the output of the receiver to complement Rin >=22k. For the Receiver I would aim for Rin>=50k to complement all transmitters that have >=330nF as the DC blocker. I also recommend that the DC blocker caps have an optional bypass facility so that two series caps are never implemented.
The most expensive of all of these components is the overly large 10uF foil cap. I would compromise and only fit this to the last item before the Power Amp assuming Rin is going to be >=50k and probably compromise even further by adopting a by-passable 1uF cap in the transmitters assuming that the Receiver has a high pass filter that can pass all the audio bandwidth that the receiver is capable of operating with.
The standard Passive Filter Formula applies. F-3dB = 1 / [2 Pi R C)
if you implement a high pass filter between two components then the more predictable location is at the input of the receiver.
As you have noticed locating the High Pass at the output of the transmitter requires the builder/manufacturer to guess at what input impedances are likely to be fitted next in the sequence.
For this reason some transmitters get fitted with high value electrolytics to allow bass to get through to all reasonable receiver impedances. I have seen 220uF Non-Polar electrolytics used for just this eventuality.
You as builder can decide that all your receiver equipment will have an input impedance that is compatible with all your transmitting equipment. Similarly you can choose the transmitters' DC blocking cap values to be compatible with all your receiving equipment. You can also decide what quality of DC blocking cap to use or even whether it is needed. All the choices are your's.
If I were being asked for typical values, I would tend towards 10uF polypropylene film & metal foil on the output of the receiver to complement Rin >=22k. For the Receiver I would aim for Rin>=50k to complement all transmitters that have >=330nF as the DC blocker. I also recommend that the DC blocker caps have an optional bypass facility so that two series caps are never implemented.
The most expensive of all of these components is the overly large 10uF foil cap. I would compromise and only fit this to the last item before the Power Amp assuming Rin is going to be >=50k and probably compromise even further by adopting a by-passable 1uF cap in the transmitters assuming that the Receiver has a high pass filter that can pass all the audio bandwidth that the receiver is capable of operating with.
The standard Passive Filter Formula applies. F-3dB = 1 / [2 Pi R C)
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Right on the dot
As per usual, Andrew, you've answered not only accurately, but eloquently (are you a professor?).Thanks for a spot-on tutorial!
My Potentiometer happens to be 50K Ohms, and I already have a couple 10uf Auricaps I've been saving for such an occasion.
My transmitters already have bypassable caps or output servos, so there is no problem.
Let's see... 330nf (that's 0.33uf, right?).
So, all I need do now is to calculate the value for C100 based upon R102(1k Ohm), or the 50K Ohm pot -which was it?
I have a small assortment of film/foil KP1832's, )including 0.082, 0.047 and 0.27uf) that I can use, depending on which resistance. 😎
Thanks again!
-Chas
As per usual, Andrew, you've answered not only accurately, but eloquently (are you a professor?).Thanks for a spot-on tutorial!
My Potentiometer happens to be 50K Ohms, and I already have a couple 10uf Auricaps I've been saving for such an occasion.
My transmitters already have bypassable caps or output servos, so there is no problem.
Let's see... 330nf (that's 0.33uf, right?).
So, all I need do now is to calculate the value for C100 based upon R102(1k Ohm), or the 50K Ohm pot -which was it?
I have a small assortment of film/foil KP1832's, )including 0.082, 0.047 and 0.27uf) that I can use, depending on which resistance. 😎
Thanks again!
-Chas
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hi, just a question from a french noob, is it possible to put a 30VA 12Vac trany in place of a 18Vac?
is it enough?
thanks
is it enough?
thanks
Hi, I didn;t see this, sorry for the late reply.
Pity no-one else jumped in.
I am not a Professor, I am not even a PhD, nor lecturer.
I am an Engineer with a little bit of school education up my sleeve.
multipliers:
n= 10^-9
u= 10^-6
so, 330nF = 0.33uF = 0u33F
A high pass (HP) filter (blocks DC and attenuates low frequencies) is series C followed by R to Signal Ground (SG).
A low pass (LP) filter (attenuates high frequencies) is series capacitor followed by R to SG.
Your Power Amplifier should have both. So, expect to see Series R+ series C, followed by R//C to SG.
Now pick out the HP components and pick out the LP components and calculate the component values for the required passband.
I believe I can answer this one. 12vac when rectified should be approximately 16-17vdc across the filter caps, which is within 10% of the 18volts specified.
I understand the B1 circuit draws very little current (<50mA), so the transformer you have chosen should work fine.
Good luck,
-Chas
p.s. Thanks again, Andrew.
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I think Andrew meant a series resistor followed by a capacitor to SG.
Attachments
I'm still trying to figure out what a smaller or larger value for C100/200 would do to the frequency response. Following the signal, I see a 25k pot to signal ground, followed by a 1k resistor and a 1uF capacitor in series. Can I conclude that this is a high pass filter with a cut off frequency of about 6.4Hz? And by raising the value of C100, the cut off frequency would be lower? (1.9Hz for C100=3.3uF)
Thago,
post a pic with the components labeled.
We can then show you which components do what.
post a pic with the components labeled.
We can then show you which components do what.
C100/200 do form a a high pass filter, but that's not why they're there... They isolate the DC at the gates of Q100/200 from the input. Also, the high-pass filter cutoff is based on C100/200, R103/203 and the input impedance of Q100/200...
Roscoe
Roscoe
Yes, as pointed out C100 R103 form a high pass filter.
There is no low pass filter in the B1. RF is fed in and the buffer has to cope with it.
F-3dB of the HP is ~0.16Hz & F-1dB ~0.3Hz.
If the effective Zin of the actual jFET were also 1M @ very low frequencies, then the F-1dB would move up to 0.6Hz.
You could change C100 to 100nF for F-1dB ~ 3 to 6Hz.
Whether you could hear any change in the low bass can only be tested by listening. Measuring will simply confirm that >20Hz has <0.05dB of change.
100nF Teflon might sound better than 1uF MKT, in the rest of the audio spectrum. Again listening is the only way to tell.
There is no low pass filter in the B1. RF is fed in and the buffer has to cope with it.
F-3dB of the HP is ~0.16Hz & F-1dB ~0.3Hz.
If the effective Zin of the actual jFET were also 1M @ very low frequencies, then the F-1dB would move up to 0.6Hz.
You could change C100 to 100nF for F-1dB ~ 3 to 6Hz.
Whether you could hear any change in the low bass can only be tested by listening. Measuring will simply confirm that >20Hz has <0.05dB of change.
100nF Teflon might sound better than 1uF MKT, in the rest of the audio spectrum. Again listening is the only way to tell.
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Well, no designed-in LP filter, R102/202 and Cgs of Q100/200 do form a LP filter, but it's, umm, not real close to the audio band ;-)
Roscoe
Roscoe
Hi Andrew
Thank you for the clear explanation, I like to know what I'm doing.
So with lower values for C100 F-1db will be higher, with higher values F-1dB will be lower. As I have some Jantzen Silver Caps 3.3uF lying around, using them would cause a lower F-1dB, thus a larger bandwith. Is this desirable, or are there any contra indications for using such high values for C100?
I doubt there's any real advantage to lowering the F-1 when it's already <1Hz. Generally, smaller caps of the same type tend to sound better than larger ones. Of course, there's something to be said for using what you've already got!
Roscoe
Hi Roscoe
Generally, smaller caps of the same type tend to sound better than larger ones That's what I was already afraid of🙄 I guess I'll have to wait for another opportunity to use the Jantzen caps, we don't want to compromise on sound quality don't we?
Generally, smaller caps of the same type tend to sound better than larger ones That's what I was already afraid of🙄 I guess I'll have to wait for another opportunity to use the Jantzen caps, we don't want to compromise on sound quality don't we?
Go ahead and build it. You can always change the caps later, then you'll know what sounds better TO YOU in YOUR system, which is the only thing that really matters anyway.
Roscoe
Roscoe
I have access to an oscilloscope and want to try lowering the output impedance. What points should I measure to determine if there is oscillation with the lower resistor values?