Hi all, is the application of a LL1540 from Lundahl a good way to increase the input impedance of a power amplifier? I wish to keep the value of the output capacitor in the previous stage at a low level while keeping the bass on a high level
The LL1540 datasheet shows the sample application. Could I increase the values to 100K? This is for a subwoofer application so no highs in the signal path
The power amp has a standard input impedance of 3600 ohms. I wish to increase this to at least above 30K or so
The LL1540 datasheet shows the sample application. Could I increase the values to 100K? This is for a subwoofer application so no highs in the signal path
The power amp has a standard input impedance of 3600 ohms. I wish to increase this to at least above 30K or so
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these 12k resistors (or 100k if you replace them) will be in parallel with the 3600 ohms of the amplifier. If you remove these resistors you will remain with the 3600 ohms only.
in 1:1 configuration the LL1540 will transmit the load 1:1 on the secondary to the primary, so you will have 3600 ohms there as well. If you wire the LL1540 in step down 2:1 (primary in series, secondary in parallel), it will have a 4:1 impedance ratio, so the 3600 on the secondary will be reflected as 14400 (4x3600) on the primary. Of course you lose halve the voltage (attenuation of 6dB).
the transformers may however require some taming (that is why the 12k resistors are there) - best tested and adjusted under your specific conditions.
in 1:1 configuration the LL1540 will transmit the load 1:1 on the secondary to the primary, so you will have 3600 ohms there as well. If you wire the LL1540 in step down 2:1 (primary in series, secondary in parallel), it will have a 4:1 impedance ratio, so the 3600 on the secondary will be reflected as 14400 (4x3600) on the primary. Of course you lose halve the voltage (attenuation of 6dB).
the transformers may however require some taming (that is why the 12k resistors are there) - best tested and adjusted under your specific conditions.
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Increasing the impedance of the circuit in which a transformer appears will shift the LF rolloff upwards. Is this OK?
Yes - I think - this is OK because there is a buffer stage after the filter stage. So there will be no shift if LF roll-off
Yes a buffer is probably needed or I need to switch the power amplifier - bummer
I have bought a pair of these IRS400SMPS | Connex Electronic without noticing the impedance (3k6). These should replace a pair of UCD400 (100K impedance) which are defective now
The Connex impledance is so low that the output cap needs to be 30uF to reach 20 hz which is my goal for the subwoofer. It may be that I go this way (buy caps) because of simplicity. Or buy another pair of UCD400s and sell the Connex
I have bought a pair of these IRS400SMPS | Connex Electronic without noticing the impedance (3k6). These should replace a pair of UCD400 (100K impedance) which are defective now
The Connex impledance is so low that the output cap needs to be 30uF to reach 20 hz which is my goal for the subwoofer. It may be that I go this way (buy caps) because of simplicity. Or buy another pair of UCD400s and sell the Connex
But it would need to be a buffer which does not require an output cap right otherwise the problem persists. Any suggestions? This is a single ended amplifier
Also I want to integrate a potentiometer (stepped pot 50K). My idea would be to put this before the buffer (which thus needs a high input impedance)
Also I want to integrate a potentiometer (stepped pot 50K). My idea would be to put this before the buffer (which thus needs a high input impedance)
Indeed, a buffer that can be DC coupled to the input of the amp: an opamp would do!
A question: have you checked if there is a cap at the input of the amp?
You can use the stepped attenuator before the opamp, ad it has a very high input impedance. Add a "big" resistor, say 220k, from wiper to gnd to avoid plops when changing volume.
A question: have you checked if there is a cap at the input of the amp?
You can use the stepped attenuator before the opamp, ad it has a very high input impedance. Add a "big" resistor, say 220k, from wiper to gnd to avoid plops when changing volume.
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...impedance (3k6). ... cap needs to be 30uF to reach 20 hz which is my goal for the subwoofer....
30uFd into 3.6k is 1.5Hz @ -3dB, 3Hz @ -1dB, 6Hz @ -0.5dB.
_I_ would try any good 5uFd cap before I would put a transformer in there. A C and an L together form a resonance. If you do not do your math, it may be a slight slump or a huge peak. A 47uFd bipolar electrolytic or a 6uFd motor-run capacitor should do fine for a lot less cash than a transformer.
I plan to have the following order:
> Pre amp (ultrapath transformer coupled)
> Tube based filter with 1uF at the end
> Stepped attenuator 50K
> Opamp buffer with some gain (no cap at the end), powered by the extra 2*18V on the power amp
> Power amp (have no answer from Connext but I guess the have implemented an input cap)
So no reluctance but would prefer to use hi Q caps only and prefer low value caps
> Pre amp (ultrapath transformer coupled)
> Tube based filter with 1uF at the end
> Stepped attenuator 50K
> Opamp buffer with some gain (no cap at the end), powered by the extra 2*18V on the power amp
> Power amp (have no answer from Connext but I guess the have implemented an input cap)
So no reluctance but would prefer to use hi Q caps only and prefer low value caps
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30uFd into 3.6k is 1.5Hz @ -3dB, 3Hz @ -1dB, 6Hz @ -0.5dB.
I used this calculator:
Coupling Capacitor Calculator by V-Cap
They say:
3db Frequency (Hz): 1.47
Optimal low frequency response (Hz): 14.74
For 20hz I would need indeed "only" 22uF:
-3db Frequency (Hz): 2.01
Optimal low frequency response (Hz): 20.10
Preferring low value caps and hearing bass are two conflicting requirements. You choose.
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Faradaphobia is a horrible disease, because it forces its sufferers into taking all sort of peculiar actions to avoid the source of their pain. It is curable, but it takes a while and the occasional relapse is likely.
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