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jev 8th January 2019 10:57 AM

Increase input impedance with LL1540?
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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

DF96 8th January 2019 12:15 PM

Increasing the impedance of the circuit in which a transformer appears will shift the LF rolloff upwards. Is this OK?

ErikdeBest 8th January 2019 12:34 PM

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.

jev 8th January 2019 01:00 PM

OK yes of course these will be in parallel so only lower the impedance in 1:1. Thanks for your replies

jev 8th January 2019 01:17 PM


Originally Posted by DF96 (
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

DF96 8th January 2019 01:45 PM

I have had a quick look at the datasheet. For your application, with a 3.6k load, you don't need the resistors at all. So you will get an impedance of 14.4k, as Erik says.

Why not use a buffer?

jev 8th January 2019 02:26 PM

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

ErikdeBest 8th January 2019 03:19 PM

Juat saw that it has +-18V supply, so a buffer is a possibility indeed!

jev 8th January 2019 03:39 PM

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)

ErikdeBest 8th January 2019 04:50 PM

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.

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