Subsaver FAQ

This FAQ is for both the Butterworth high-pass filter and it's associated subsaver calculator

Q. What is the maximum gain that can be achieved?

This depends on the supply and input voltages

To maintain linearity, the output should not exceed 80% of the supply voltage.
Whilst it is possible to supply this circuit with 12V or even lower, the recommended value of 15V gives an allowable output of 12V peak or 8.49Vrms.

If driving from a "pro level" source, the operating level should be around 1.23 Vrms
Since Gain= 20 log (Vout / Vin) the maximum gain would be 16.8dB

If driving from a "consumer level" source, the operating level should be around 0.3 Vrms
In this case the maximum gain would be 29dB.
Since the output at this gain would be more than is needed to drive a "pro" amp to full output, the maximum gain for the calculator has been set to 20dB
Thanks go to Bob Worthington for this erudite answer!

Q. How can the gain be made variable?

Subject to the input voltage constraints listed in the previous question, a range of 1.2dB gain thru 20dB gain can be achieved by replacing R1 with a fixed 1.6k resistor in series with a 100k potentiometer, wired as follows:

Mod to add variable gain

Q. How can the output be made balanced?

Whilst this is departing from the minimilist design, a balanced output stage can be tacked on the end using a DRV134 IC
The 10uf caps are non-polarised electrolytics. For more info, see the manufacturer's data sheet (pdf)

This is basically the same partial circuit used by Chasw98 and ThomasW in the H@LF design

Mod to add balanced output

Q. Why not balance the impedances into the first op-amp?

Input stage

It is generally good practice to balance the input impedances to an op-amp where possible to reduce DC offset at the output. To achieve this in the buffer stage shown above, Rin should equal the value obtained if R1 and R2 were in parallel.

This was not done for three reasons....

• The buffer stage is coupled to the following stage by capacitors. This prevents any DC offset from being carried forward
• By making R2 fixed, the calculator no longer needs to ask the user for the value of Rin and R2, simplifying the program interface
• R2 has a 0.01uf bypass capacitor in parallel, causing a response pole calculated by f=1/(2*pi*R*C). It has been suggested that keeping this above 1kz is a good idea, which requires R2 to be no more than 15k. This is the value used in the calculator and recommended in the design notes

Q. What are the limits on values for each field in the calculator?