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Old 18th Nov 2018, 11:48 pm   #1
1100 man
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Default Getting a BA3308 Auto level chip to work!

Hi,
I have not used one of these before, but the data sheet looked quite sensible so I ordered one and have been having a play with it.

I have connected it as per the circuit below, taken from the Rohm data sheet. The voltage gain (Gv) is given by the ratio of R5 to R3, which is 18K over 39R which is 461.
So with a 300mv input signal, what would be the expected output voltage? Obviously not 300mv x 461 = 138V!

In fact with the above values and 300mv input, I only get about 150mv out. I have to massively increase the 18K to 68K to get an output of 900mv.

The data sheet also says that the auto level action starts when the output exceeds 750mv or so. Increasing the value of the 18K still further so that the output increases to 1.2V, I still don't get any ALC action.

I don't doubt I've mis- interpreted the data sheet in some way- past experience would suggest that is most likely!

So what am I doing wrong?

Just to add, I am only using CH1, Ch2 is not connected. Also, the output pin (3) is only connected to the scope.

The data sheet is here: http://rtellason.com/chipdata/ba3308.pdf

Many thanks
Nick
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Old 19th Nov 2018, 4:29 pm   #2
orbanp1
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Default Re: Getting a BA3308 Auto level chip to work!

Hi Nick,

Playing with that chip must be fun!

Just a reminder, that chip was developed for tape playback amplification, the output voltage from those tape heads is in the mV range!
As you mentioned, the gain with those resistor values is about 53dB!
So using 300mV as input signal sounds awfully high!
Do test the circuit with smaller input signal!

EDIT:
Alternately, you could reduce the gain to 10 (20dB), or even lower, like 3 (~10dB), by changing the value of R3 from 39Ohm to 1.8kOhm or to 6.2kOhm. That would give you input signal levels closer to "line level" signals.

See the ALC characteristics in Fig. 17 in the datasheet!
For reference, here is a definition of dBV:
https://en.wikipedia.org/wiki/Decibel#Voltage

Also double check the series input resistor value, R1, 3.9kOhm. For the electronic voltage divider (ALC function) to work properly that value needs to be close to what they suggest in the datasheet.

Good luck, Peter

Last edited by orbanp1; 19th Nov 2018 at 4:51 pm.
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Old 19th Nov 2018, 4:32 pm   #3
John_BS
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Default Re: Getting a BA3308 Auto level chip to work!

Do you have a scope to monitor this circuit?

Here are some possibilities!

1. the unused half is not happy being left unconnected and is oscillating (hence applying agc to the driven half)

2. Ditto but just being left hanging has upset the alc section.

3. Finger trouble: have you checked the dc's on all pins?

Try starting with the circuit as a simple amplifier (short pin 5 to ground) and see if it then has the expected gain (c. 450) : so you only want about 1mV rms input.

John
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Old 19th Nov 2018, 11:49 pm   #4
1100 man
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Default Re: Getting a BA3308 Auto level chip to work!

Quote:
Originally Posted by orbanp1 View Post

Just a reminder, that chip was developed for tape playback amplification, the output voltage from those tape heads is in the mV range!
Hi Peter,
Thanks for the reply. The data sheet shows the chip being used as a record or playback amplifier. As far as I can tell, the ALC would only be active in record mode. They show a complete record/ playback circuit for a tape recorder. The input in record says ' mic or radio' so must be much greater than a few mv.

However, I will do some more experiments with lower level input signals.

Hi John,
I am monitoring what's going in and out with a 'scope. I had originally connected the unused channel inputs to ground via 10k resistors in case it was upsetting the ALC circuit. This didn't make any difference.
I measured the input resistor which was 3K9. I will try disabling the ALC by shorting pin 5 to ground and see what happens.

Many thanks
Nick
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