Designing and building a speaker start delay circuit

Gold plated rear input and output banana sockets.

Did I need to make this?

Earlier this year I purchased a hybrid EF95/6AK5 hybrid tube amp from Amazon. The make and model was Fosi Audio T20.

The amp cost about £70 and is a great little amp for driving loudspeakers or listening via headphones, the input is fed through the tubes and then amplified using a TI solid state amp. So obviously it's not a 100% tube amp but a hybrid one.

One thing I noticed is that there is a large amount of DC when turning on the amp, this is not good for the speakers. This can be avoided by flipping the toggle to the off position, however this keeps power to the amp on and it gets slightly warm.

Now this being something that is cheap and coming from China, I didn't trust it enough to leave it on like this all the time. So this is when I decided to make a circuit using relays to delay the speaker connection and to add in inline power switch, this meant the power is fed to my circuit as well as the amplifier.

When I originally designed the circuit I did not test it properly with the amp, the original circuit included a DC detector as well as a start delay, after having the PCB fabicated I made it and found it did not work properly. It turns out the speaker negative is not common ground, it's either isolated from the DC ground or floating, as a result the conventional DC detector circuit did not work.

So it was back to to EasyEDA and I redesigned it as a simple start delay.

Video of the completed project.

The schematic

The circuit consists of only three transistors and two power relays.
C1 is the delay capacitor, when it's charging Q2s base is deprived of current keeping it off and in turn keeping Q3 off, once C1 is charged Q2 starts conducting it turns on Q3 which in turn drives the relays.

EasyEDA Schematic

I used two resistors where R5 and R6 are, as they get a bit warm, I could have used a 1/2W resistor but I didn't have any and didn't want to order any.
I used common transistors and MPSA92 was chosen for its high voltage capability, this is because when the relays are de-energised they release their stored energy and this can build up quite a bit of a voltage on the transistors collector. Having the flyback diodes help with this and is a must. I opted for the relay diodes to be paired with a zener diode, this helps with the relay reponse times.

The choice of relay is dependant on the power being handled by them, DC is quite destructive to the relay contacts, if this was connected to a high power amp then the contacts on the relays over time may get degraded. I chose 16A Finder relays, they are overkill for my low power 50W amp, but they are transparent and look cool. It is important to connect the NC pins to speaker ground, will will prevent arcing inside the relay.

The Calculations

I don't have a formula for calculating the delay capacitor, but with a 33uF you get a 3 second delay with a 24v power supply, 22uF gives you about 2 seconds. D1 helps discharge the capacitor quickly.

Applying a positive voltage to the base of Q1 will turn off Q2 (and Q3). The relay can be a single 24v relay or in my case I've used two 12v relays in series.

The base resistors for the transistors need to be calculated to ensure they are saturated. Base resistor is calculated as follows:

I_B = Base Current
I_C = Load Current (the relays used here use about 40mA, we'll use a value of 50mA)
R_B = Base Resistor
V_IN = VCC (24v)
V_BE = Base Emitter Voltage
β = hFE

Q1 and Q2

β = from specsheet: 200 (this is the gain and is also known as hFE)
V_BE = 0.7v

Q3 - MPSA92

β = from specsheet: 25
V_BE = 0.7v

The base resistors for Q1 and Q2 are not too important as their load current is very low and they have a very high gain, so we can just choose a 100kΩ resistor which will be more than enough to saturate them.

To calculate R5 and R6 for Q3 we first need to work out the required base current (I_B), to guarantee full saturation I added an extra 30% by multiplying the result by 1.3:

I_C 0.05 mA
I_B = ______ x 1.3 = __________ x 1.3 = 0.0026 mA
β 25

Now that we have the I_B we now can work out R_B (R5 and R6):

V_IN - V_BE 24 v - 0.7 v
R_B = ___________ = _____________ = 8.96 kΩ
I_B 0.0026 mA

The closest value is 9.1kΩ you can use a single resistor, or in my case I've use two 18kΩ resistors in parallel to achieve 9kΩ.

The PCB

For the PCB design I used EasyEDA, for a free tool I find it very powerful and it has improved over time.

The front of the PCB.

The dimensions of the PCB is 64mm x 46mm.
I opted for suface mount resistors and diodes. All the trace widths for the relays are wide enough to handle high current if needed.

JST PH and XH connectors are used for the LEDs and power switch, with a micro toggle switch used to turn off/on the relay activation LED.

A 6 way PCB mouted screw terminal is used to connect up the speakers and amplifier.

A PCB was also used to hold all the connections at the rear.

The rear input/output panel.

Power being fed in to the circuit is immediately fed back out to the amplifier.

The Build

As I'm not mass producing this and with my limited amount of tools, I prefer to use 5mm or 3mm 9T20 Perspex sheets. It's easy to work with and very easy to cut and drill. It also looks cool when complete, as you can see insides.

PCB mounted on the perspex panels.

I've tried working with wood, but it's hard work and requires more specialist tools. I don't like metal cases so would not consider using it.

Once my perspex sheets have been cut down to size, I sand them using wet fine grit (1200) sandpaper to get the smooth edges. I then use a drill press to get the holes made for the PCB mounts and the spacers.
The spacers are knurled aluminium used for RC toys.

Bill of Materials

Name	Designator	Qty	Manufacturer Part	Manufacturer	Supplier	Supplier Part
KF301 5.0 6P	AMP/SPEAKER	1	PA001-6P	HIWA	LCSC	C173288
ZMM24	ZD2,ZD1	2	ZMM24	SEMTECH	LCSC	C8074
FINDER 40.61.7.012.0000	RELAY.LEFT RELAY.RIGHT	2	182-9781	Finder	RS-Online	40.61.7.012.0000
1N4002W	D3,D2	2	1N4002W	Jingdao	LCSC	C169542
XH-2A	24V	1	XH-2A red RHOS	BOOMELE	LCSC	C24192
10K	R8,R7,R2	3	RC1206FR-0710KL	YAGEO	LCSC	C132649
91K	R4	1	RC1206JR-0791KL	YAGEO	LCSC	C137108
BC546BTA	Q2,Q1	2	BC546BTA	ON Semicon	LCSC	C258143
18K	R6,R5	2	RC1206FR-0718KL	YAGEO	LCSC	C137379
PH-2AK	ACTIVE,PLED	2	PH-2AK	BOOMELE	LCSC	C8886
MPSA92-AT/P	Q3	1	MPSA92-AT/P	KEC	LCSC	C112896
33uF	C1	1	PLV1H330MCL1	Nichicon	RS-Online	739-5396
XH-2A	SWITCH	1	xh-2ayellow	BOOMELE	LCSC	C57141
MSK-12D19	NO	1	K3-1260D-F1	Korean Hroparts Elec	LCSC	C223843
100K	R3,R1	2	RC1206FR-07100KL	YAGEO	LCSC	C137393
1N4148W-7-F	D1	1	1N4148W-7-F	DIODES	LCSC	C83528