TL072 based Tape head preamp - Page 1
Forum section: electronics / projects

author: Wombat
author_rank: Contributor
timestamp: August 05, 2022, 12:45:00 pm
content: Hi Everyone,I am trying to design a single rail op-amp based tape head preamp and need some help on what I am doing wrong.The preamp is supposed to amplify the tape head signal to line level for playback.I breadboarded it but got nothing out of it but a square waveIf anyone can offer a few pointers I would greatly appreciate it!Wombat

author: Andy Watson
author_rank: Super Contributor
timestamp: August 05, 2022, 01:04:18 pm
content: The tape head will short-out the bias for the first op-amp. It needs to have some DC blocking.

author: Zero999
author_rank: Super Contributor
timestamp: August 05, 2022, 01:11:20 pm
content: The bias point isn't filtered, so it will pick up noise from the power supply, which can be a problem, unless it's run off a very clean power supply i.e. a battery without any noisy load connected.Refer to the schematics below for how to properly design a single supply op-amp circuit.Credit: audioguru for the original.

author: magic
author_rank: Super Contributor
timestamp: August 05, 2022, 01:24:14 pm
content: With TL072 it may be a good idea to treat BAT+ as ground and BAT- as a negative rail and filter all voltages with a capacitor to BAT+. This way, when the circuit powers up, it avoids dragging the input through the cursed region near the negative rail and triggering big output transients due to phase reversal.(This assumes that the opamps are actual TL072 and not relabeled LM358 from AliBay)

author: Zero999
author_rank: Super Contributor
timestamp: August 05, 2022, 07:20:17 pm
content: That's a good idea. Here are the basic op-amp circuits with a negative supply and all signals referenced to the 0V which is connected to the positive side of the battery.

author: Wombat
author_rank: Contributor
timestamp: August 07, 2022, 02:45:01 am
content: Thanks for the input everyone!Wow that is super confusing, so in a single rail sense, Op Amp Ground becomes +12V and -12V is the battery ground???Also do I need a coupling capacitor between op amp stages?So my tape head ground would also need to be on the +12V (and isolated from the chassis ground)?Sorry for the questions, but I have no experience with opamps on single rail supplies...

author: andy3055
author_rank: Super Contributor
timestamp: August 07, 2022, 03:40:41 am
content: You may want to study these pages from TI:https://www.ti.com/lit/an/sboa092b/sboa092b.pdf?ts=1659802527013&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FLF353%253FCMP%253Dconv-poasamples%2526bm-verify%253DAAQAAAAF_____ymMyvOOqt5hUkAt897xaue6ZuOwI8wBC0JAtSjLd1q3eoshg0i2Hu7VpSxkytGtrlL1XZrl3SM0WNGsiuzbbvf_8ztixGAZ776PaX7-NTsS0B-N5VRzqEygaz8Md35v2YgO38BLGA3ngI3A9M19cVXfuvEb_BBfRaqCWPS0tLPPQDm1Yb8iRhn6DHd1_5SInTTdEz9j6UctGk2cmXDxXabQTlFYoCZrItWm3fNb7bJFPodBdh9wEpcunm3sIPE0J4eicByBOHjmZIQ

author: Wombat
author_rank: Contributor
timestamp: August 07, 2022, 04:05:22 am
content: Schematic updated, have yet to try this one...

author: iMo
author_rank: Super Contributor
timestamp: August 07, 2022, 07:05:29 am
content: You should ground the pos side of the 47uF capacitor.Also a good practice would be to add a resistor in series with the 10uF capacitor (sets the Gain).What is the impedance of your tape head?

author: Zero999
author_rank: Super Contributor
timestamp: August 07, 2022, 08:55:48 am
content: There were a few more errors: the AC capacitor on the inverting amplifier should be connected to 0V and a missing 0V on the potential divider.Why is the input now an inverting stage? It needs an input resistor. It was better as a non-inverting amplifier, which can have a higher input impedance.The 3k6 and 15nF capacitor make cut-off frequency 3kHz: is there a reason for this?

author: Wombat
author_rank: Contributor
timestamp: August 12, 2022, 08:20:22 am
content: It is a generic type around 150...

author: Wombat
author_rank: Contributor
timestamp: August 12, 2022, 08:29:05 am
content: Zero999,Thanks for that,It is meant to be non inverting, I messed up and flipped the op-amp position in Express schematic...The 15nf and 3.6kis meant to be the NAB circuit as described in the LM833 datasheet page 10.This is sort of like the famous RIAA circuit used in phono preamps...Will give the reworked schematic a go.Thanks very much for your help.