LA Audio Electronic SPX20 operation manual Circuit Description, Inputs 2 & 3 Trim

6.0 CIRCUIT DESCRIPTION

Left
	MIX		•
	L R	Tape Send L
	••	Mix amp	•		•
INPUT 1					MONO
			•	BALANCE
Right	Select #1	Tape Send R
TRIM
	•	Mix amp	•		•
	•
Left				•
	•	Select
		#1
				•
INPUTS 2 & 3 TRIM	Select #2	Select
		#2		•
Right		Select
	•				Input
		#3
				•	Select
		Select
		#4		•
Left
•	•	Select
		#5		•
TRIM	Select #4	Select
INPUTS 4, 5 & 6		#6
Right
•	•	~			Power
					on-off
					Delay

L R

	•		Left
	•
	DIM	On-Off #1	OUTPUT 1
	LEVEL 1
			Right
	•
	•
			Left
1	•
	LEVEL 2	On-Off #2	OUTPUT 2
2			Right

•

3

4

5
VOLUME
6
On-Off #1	HEADPHONES
	On-Off #2

•

OUTPUT 1 ON-OFF OUTPUT 2 ON-OFF

Fig 6.1.1 SPX20 block diagram showing main signal paths

INPUT 1

The input is routed through a pair of half normalled TRS jacks and into a differential amplifier. The TRIM varies the gain of the differential amplifier so ensuring maximum headroom at all gain settings. The output of this amplifier is fed via CMOS switches onto the mix buss.

INPUT 2 & 3

Inputs 2 and 3 are similar to Input 1 without the insert jacks.

INPUT 4, 5 & 6

These inputs are similar to Inputs 1-3 but are unbalanced.

The CMOS switches are controlled by the input select logic block. This consists of six set-reset latches. Pressing a select button sets its latch and sends a reset signal to all the other latches.

Signals on the mix buss are summed by two virtual earth amplifiers. These feed the TAPE RECORD output. The mixed signal also feeds the BALANCE pot, MONO and DIM switches.

OUTPUT 1 & 2 are relay switched. These relays are controlled by the OUTPUT on- off switches and the power on-off circuitry. The output amplifiers are configured as ground compensated, balanced impedance.

All inputs and outputs have RFI filtering.

Page 13