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Below is a simple buffer circuit using an isolation transformer :

enter image description here

The green trace is the output of the transformer and the blue trace is the input signal (20hz to 20Khz sine wave). As you can see the bass response is not very good (I deliberately modelled a cheap transformer)

I've heard of negative impedance drivers, but I can't find enough detail anywhere to implement one.

Can anyone tell me :

1) how to accurately model this transformer

2) how to modify my buffer to increase the bass response?

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  • \$\begingroup\$ Is this audio? How much more bass? perhaps -3dB @ 20 Hz? \$\endgroup\$ – glen_geek Dec 21 '18 at 15:01
  • \$\begingroup\$ @glen_geek in an ideal world I'd obviously like the output to exactly match the input. But any method of improving the bass response by any amount would be fine. It's for a guitar.. not hifi. \$\endgroup\$ – Richard Dec 21 '18 at 15:40
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    \$\begingroup\$ What do you get if you used the voltage source directly on the transformer primary and ditched the op-amp and push-pull stage? \$\endgroup\$ – Andy aka Dec 21 '18 at 15:51
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    \$\begingroup\$ If LTSpice model configured with DC resistance of the coil (it is invisible on the chematic), no additional resistor is needed. At this point low frequency corner Flow can be evaluated: 2*pi * Flow * L = Rdc, so for L=2H Flow will be about 70 Hz. The proper way to increase bass response is to use appropriate transformer. The chosen one has "200 Hz to 15,000 Hz range" and there is no good way to expand it down to 20 Hz. \$\endgroup\$ – Eugene K Dec 21 '18 at 19:15
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    \$\begingroup\$ You cannot use this signal transformer for Bass as the current increases with lower f and results in core saturation if you drive it at 100mW levels. It is designed for 10K impedances and telephony frequency response of 275Hz to 3.5KHz. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Dec 21 '18 at 22:15
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The drop-off of the transformer transfer, about - 3dB at ~60 Hz, neatly matches the data of the transformer, i.e. 820 Ohm and 2 H. So, no way to improve it with this transformer. So, which one should fit?
1. Typical (passive!) magnetic pickup output parameters are: resistance=3-10 kOhm, inductance=2 H and capacitance=200 nF (see e.g. ironside-guitar-pickups.co.uk). Output voltage with heavy plugging runs up to 50 mV (RMS). In your schematic the buffer takes care of these impedances, while the unity gain provides 50 mV at the input terminals of the transformer. So, that's OK.
2. Normal guitar frequencies run down to (E)=82 Hz; bass-guitars one octave lower (41 Hz). Which application is yours?: match accordingly.
3. The transformer should be suited for frequencies well below 82 (or 41) Hz, say 50 Hz for a normal guitar, without frustrating the higher frequency side. Stroll around in www.oep.co.uk, with its wide range of parameter choices at decent cost. Can be ordered from some well known on-line stores (RS, Farnell, ..). These transformers easily can handle the ~50 mV signal levels. 4. The buffer amplifier is not too nice with its output transistors closed at zero to small signals, leading to large crossover transient distortion. Try to avoid this output pair what so ever, or add a base-emitter resistor of a few 100 Ohm.
5. !!: Have no interests in any of the mentioned companies!!

@ Richard : I could not see how to 'improve' or 'overcome' the transformer properties as such!
On the NPN-PNP output stage: likely you can do without, as the opamp can deal with the transformer load. Take 100 mV_RMS max (= ~300 mV_pp) at 50 Hz, and a 2H & 800 Ohm transformer. This results in ~1 kOhm imput impedance, and so 0.3V/1kOhm = 0.3 mA_pp of current. This easily can be provided by nearly any opamp.

@ Richard : Just an addition. You may boost the gain of the buffer opamp a bit between 50 - 200 Hz. Just mount a 4.7 kOhm resistor between output and opamp negative input pin Vin_neg and ~ 2.2 kOhm between this input and ground. Put 4.7 kOhm and a ~0.3 uF capacitor in series, and mount this in parr. to the 4.7 kOhm between output and Vin_neg. For f> ~200 Hz the gain will be close to 1; for f~50 Hz the gain will be ~2. Please check my (thumb) 'calculation'!

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  • \$\begingroup\$ Thanks for this. I have available to me some 'expensive' chinese audio transformers which are suitable, I know them to be suitable because I've tested them. But they're prohibitively expensive. My intention is to make many of these units. These Triad TY-141P transformers are aaaalllllmost good enough. If I could just kick them a little at the bass end. It's a guitar, not a bass application. The crossover distortion problem is handled by the opamp feedback loop isn't it? Do I still need base emitter reisotors? \$\endgroup\$ – Richard Jan 21 at 16:01
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Try driving the transformer primary from a zero-impedance source. The output of an op-amp is ideal for this.

I have successfully used significantly crappy transformers from Mouser (TU016) with excellent results using this technique. -3 dB Frequency response improved from around 200Hz down near 25 or 30Hz.

The Mouser TU016 is a 600-600 Ohm line transformer intended for telecom or general PA System usage. Frequency response below 200 Hz is not needed.

However, we had a project that needed much better low-frequency response but did not have the budget for better transformers. The technique I mentioned above gave excellent results.

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