Practical_Electronics-May_2019

AUDIO OUT

L R

AUDIO

OUT By Jake Rothman

T

he circuit presented here

is a very small Baxandall tone

control module designed by

Mike Grindle; owner of ‘Life is Unfair

Audio Devices’ (www.facebook.com/

lifeisunfairaudio) who make and design

bespoke audio products, mainly gui-

tar pedals. This circuit is specially de-

signed as a little add-on for fuzz pedals

and other effects, but as pointed out in

Part 1, it can be used in inexpensive Hi-

Fi. (In a future issue I’ll present a more

refi ned Hi-Fi tone control using 5532

op amps.) This module is based on the

TL072/62 Bi-Fet op amp, which suits

high impedance sources like electric

guitar. However, you can use the 5532

op amp, but you will need to make a few

small mods, which I will detail below.

Note that the input bias current for

the op amp goes directly through the

bass pots wiper, which could cause

noise. This only matters with bipolar

op amps, such as the 5532 and 4558.

With Bi-Fet op amps, their bias cur-

rents are so low it is not a problem.

Note also, that with bipolar op amps,

the input bias resistor R2 should be

dropped to 220kΩ. If DC blocking

is required, put a 10μF capacitor in

series with R5 and put a 220kΩ resis-

tor across C6 to provide a DC negative

feedback path (see Fig.16).

Battery power and

op amp selection

Most guitar foot pedals work on a

+9V power rail, rather than the usual

±12V/15V op amps normally use. You

have to be careful in these situations

to get the bias point optimised. Theo-

retically, it is +4.5V; however, most op

amps fi nd it more diffi cult to swing right

down to 0V than up to V+. So, for the

TL072 it is best to bias it at 4.9V, mak-

ing R10 equal to 82kΩ (Fig.8 in Part 1).

This will give a maximum output of

6.8V pk-pk, as opposed to 6V pk-pk

with 4.5V of bias. The current consump-

tion is 2.8mA. The low power (0.5mA)

TL062 gives more voltage swing, pro-

viding 7.8V with R10 = 91kΩ. The 5532

is not really suitable for 9V because the

negative cycle drive is reduced under

load unless it is biased to +6V. It’s much

better to run a 5532 at +24V.

Stability

If you opt for the 5532 approach then

note that there is a specifi c problem

Backing Baxandall – Part 2

Article by Jake Rothman

PCB and circuit by Mike Grindle

Bass

Treble

R4

6.8kΩ

R3

2.2kΩ

R7

47Ω

R6

220kΩ

Input

from

buffer

Output

DC block

cw

cw

VR1

10kΩ

Lin

VR2

10kΩ

Lin

C2

150nF

C4

15nF

C5

15nF

C3

150nF

R5

1kΩ

C6

6.8nF

0V

0V

R1

1kΩ

R2

1kΩ

C8

330pF

C10

22pF

+15V

3

2

7

(^48)
4
6
DC path
–15V
R8
220kΩ
C1
10μF
C9
47μF

• 
  


• 
  


• 
  

• 
  

IC1

5534

47μF

Bipolar

Fig.15. Mike Grindle’s PCB for the Baxandall circuit given in Fig.8

in Part 1 last month.

Fig.16. Baxandall circuit using 5532 op amp. Note compensation

network (R5, C6), DC feedback path (R6) and lower impedances.

with high-frequency stability when it is

used at gains below unity (as occurs in

tone controls on the cut settings). This

is caused by a reduced phase margin

with the internal feedback compensa-

tion. One way round this is to use the

single version – the 5534 – and over

compensate it by pushing C10 from

22pF to 39pF (Fig.16), which will re-

duce slew rate. A better method is to

put a series RC network from the in-

verting input to ground, see R5-C6 in

Fig.16. This will increase noise gain,

but at inaudible frequencies. Finally,

long leads to the pots can cause insta-

bility, this can be fi xed by increasing

the feedback phase-lead capacitor (C8)

to say, 470pF (max).

The component values shown in

Fig.8 (Part 1 – see last month) are op-

timised for guitar, with the pivot point

at 800Hz rather than the normal 1kHz

Hi-Fi fi gure. The curves are shown

in Fig.17. C6 is increased from 68pF

to 2.2nF on the guitar version to pre-

vent the treble rise from going above

10kHz. The same with the output

capacitor (C7), which rolls off below

50Hz, whereas for Hi-Fi it should be

upped to 10μF.