dt770 DT 770's drivers rattle under relatively extreme conditions.

[just_dave]

13 hours ago, Nimrodor said:

Those are almost definitely false or manipulated in some way. 7.3Vrms is physically impossible using their parts. Most likely they are summing the outputs of both channels when giving their power output spec, not using a standard test signal for power measurements, or simply don't care about lying.

 

Measurements show that the voltage swing is limited to what would be expected for a 12V device.

Model a pot in series with a 0Ω output source and calculate the resistor noise. At full volume the network's effective impedance is 0.

The noise was higher even with no signal being played?

 

Raising the volume also doesn't strictly increase input impedance; it depends on the input device's input impedance. For instance, if there is a 10kΩ impedance looking into the BJT's base, then input impedance is lower at full volume (10kΩ||100kΩ) than at half (50kΩ+50kΩ||10kΩ).

There was never any signal played when i tested the noise.

Here are schematics from the Harman amp.

I am going off from what i was told caused the hissing issue.

The resistors at the base of Q501 are 140K total. 

The 100K volume pot with it's 4.7K resistor is like 4900 ohms total.

Now what? What's the total input impedance ?

[Spuriae]

10 hours ago, thicc_boi said:

There was never any signal played when i tested the noise.

Here are schematics from the Harman amp.

I am going off from what i was told caused the hissing issue.

The resistors at the base of Q501 are 140K total. 

The 100K volume pot with it's 4.7K resistor is like 4900 ohms total.

Now what? What's the total input impedance ?

If the noise is annoyingly high, I doubt it's due to the resistors. What is the spectrum like? White noise, mains hum?

 

Anyways... how is the pot actually wired? Looking at the schematic I'd guess that the 4.7kΩ is a shunt on the output of the pot to give it a logarithmic response. That would be small enough to determine the minimum input impedance on its own (all other impedances are an order of magnitude or more larger).

 

Assuming VR501 was set to max and β for the transistors was 250 (datasheet for 2SC2320-F says the range is 250-500), I get:

   

Volume	Input Impedance	Johnson Noise Resistance at Input
10%	93375.19539	2945.884062
20%	83922.13269	3438.37805
30%	74150.88477	3624.000267
40%	64276.54557	3704.229783
50%	54355.98933	3727.796058
60%	44410.75288	3705.786395
70%	34450.79081	3627.887266
80%	24481.33866	3447.389712
90%	14505.41304	2973.511793
100%	4524.874526	305.9330343

[just_dave]

1 hour ago, Nimrodor said:

If the noise is annoyingly high, I doubt it's due to the resistors. What is the spectrum like? White noise, mains hum?

 

Anyways... how is the pot actually wired? Looking at the schematic I'd guess that the 4.7kΩ is a shunt on the output of the pot to give it a logarithmic response. That would be small enough to determine the minimum input impedance on its own (all other impedances are an order of magnitude or more larger).

 

Assuming VR501 was set to max and β for the transistors was 250 (datasheet for 2SC2320-F says the range is 250-500), I get:

   

Volume	Input Impedance	Johnson Noise Resistance at Input
10%	93375.19539	2945.884062
20%	83922.13269	3438.37805
30%	74150.88477	3624.000267
40%	64276.54557	3704.229783
50%	54355.98933	3727.796058
60%	44410.75288	3705.786395
70%	34450.79081	3627.887266
80%	24481.33866	3447.389712
90%	14505.41304	2973.511793
100%	4524.874526	305.9330343

It was white noise-ish. The resistors were on the input iirc.

Yep, i was also told it was most likely caused by old transistors.

The pot itself was logarithmic.

[Spuriae]

8 hours ago, thicc_boi said:

It was white noise-ish. The resistors were on the input iirc.

Yep, i was also told it was most likely caused by old transistors.

The pot itself was logarithmic.

Slightly lower pitch, like pink noise?

 

Either way, the amplifier shouldn't act any differently when there is no signal, regardless of the position of the potentiometer, unless something is causing the input impedance at the base to be lower than expected. It's probably the transistors.

[just_dave]

23 hours ago, Nimrodor said:

Slightly lower pitch, like pink noise?

 

Either way, the amplifier shouldn't act any differently when there is no signal, regardless of the position of the potentiometer, unless something is causing the input impedance at the base to be lower than expected. It's probably the transistors.

yep, probably the transistors.

I rigged out that amplifier a while ago and i use just the power amp board. It's very clean with great sound and a lot of power, so it was not worth trashing the amp just because of bad tone controls.