Trying to figure out some preamp/amp compatibility issues

Thread Starter

askDIY

Joined Oct 18, 2023
17
I built this preamp:
1710207147389.png



and I'm planning on attaching it to this amp circuit via terminal connectors:
1710207175658.png

to drive a Jensen 25-watt 8-ohm speaker. Currently there seem to be issues with the simulations as you can see:
1710207220007.png


This is being fed 0.2 Vpk. If the input for the current design exceeds 0.2 Vpk it distorts. The preamp is outputting 2.3 Vpk which is way too high. I'm having trouble figuring out exactly what the solution could be?
 

Audioguru again

Joined Oct 21, 2019
6,758
Your preamp and power amp have no extremely important power supply voltage VCC that determines clipping levels and output power.
The datasheet for the LM1875 shows that a 55V supply will allow it to produce 25W at low distortion into 8 ohms but the maximum allowed supply for the TL072 is only 36V.
 

Thread Starter

askDIY

Joined Oct 18, 2023
17
Your preamp and power amp have no extremely important power supply voltage VCC that determines clipping levels and output power.
The datasheet for the LM1875 shows that a 55V supply will allow it to produce 25W at low distortion into 8 ohms but the maximum allowed supply for the TL072 is only 36V.
How big of an issue is this?
 

crutschow

Joined Mar 14, 2008
34,680
If you look at the LM1875 data sheet (you should try that) there is a graph that shows output power vs. supply voltage (below):
Note that's for a ± dual supply. A single supply would be double that voltage.
So select the desired power output, and that will tell you what supply voltage you need.

The power supply(s) total rating (volts x amps) should be at least 25% above the peak power to the speaker.

1710260951140.png
 

Thread Starter

askDIY

Joined Oct 18, 2023
17
If you look at the LM1875 data sheet (you should try that) there is a graph that shows output power vs. supply voltage (below):
Note that's for a ± dual supply. A single supply would be double that voltage.
So select the desired power output, and that will tell you what supply voltage you need.

The power supply(s) total rating (volts x amps) should be at least 25% above the peak power to the speaker.

View attachment 317470
View attachment 317494
1710280884159.png
with a supply of 50 Volts, while there is no distortion and I don't hit a saturation point, the lm1875 is pumping out peak 25 watts, and the load is pumping peak 52 watts, which would result in the circuit blowing up...
 

Audioguru again

Joined Oct 21, 2019
6,758
71% of 25W is 17.7W. Since the sensitivity of our hearing is logarithmic then 17.7W sounds almost as loud as 25W.
Turn down the gain control to eliminate clipping.
 

Thread Starter

askDIY

Joined Oct 18, 2023
17
71% of 25W is 17.7W. Since the sensitivity of our hearing is logarithmic then 17.7W sounds almost as loud as 25W.
Turn down the gain control to eliminate clipping.
1710287103884.png
Even if gain is turned down, in order to power the speaker, I need 50V with a single supply, however, when simulated, even though there is no distortion and I don't hit a saturation point, the lm1875 is pumping out peak 25 watts, and the load is pumping peak 52 watts, which would result in the circuit blowing up...
 

LowQCab

Joined Nov 6, 2012
4,196
First off, use a LM3886TF Audio-Amplifier-Chip instead of the LM1875.
This Chip can withstand a lot of abuse, and comes in a larger,
completely Isolated-Package that can dissipate substantially more HEAT.
And, it has slightly more Output-Power for the same Supply-Voltage.

The Components that You will need to pick for an appropriate
Power-Supply should be the first order of business.

To make things easy, and safe, and dirt-cheap,
use 2 SMPSs from a couple of used LapTop-Computers from your local Computer Repair-Shop.
This will create a very reliable Plus and Minus ~20-Volts Power-Supply,
and will require 2- 5-Volt Zener-Diodes to reduce the Voltage-Rails for the Pre-Amp Power-Supply,
and several Large-Capacitors to eliminate any odd Electrical-Noises.
These generic LapTop-SMPSs are also available new if You prefer, and are quite inexpensive.

This will get You close to a solid ~20-Watts RMS of Clean-Power, with no-Clipping,
into a single 8-Ohm-Speaker.
This setup will also drive a second 8-Ohm-Speaker, ( 4-Ohm-Total-Load ),
at the same time, while producing very close to ~40-Watts of RMS-Power.

You will also need an Aluminum-Box to build it in,
and to provide the necessary Heat-Sinking to keep it cool and alive.
.
.
.


 

Thread Starter

askDIY

Joined Oct 18, 2023
17
First off, use a LM3886TF Audio-Amplifier-Chip instead of the LM1875.
This Chip can withstand a lot of abuse, and comes in a larger,
completely Isolated-Package that can dissipate substantially more HEAT.
And, it has slightly more Output-Power for the same Supply-Voltage.

The Components that You will need to pick for an appropriate
Power-Supply should be the first order of business.

To make things easy, and safe, and dirt-cheap,
use 2 SMPSs from a couple of used LapTop-Computers from your local Computer Repair-Shop.
This will create a very reliable Plus and Minus ~20-Volts Power-Supply,
and will require 2- 5-Volt Zener-Diodes to reduce the Voltage-Rails for the Pre-Amp Power-Supply,
and several Large-Capacitors to eliminate any odd Electrical-Noises.
These generic LapTop-SMPSs are also available new if You prefer, and are quite inexpensive.

This will get You close to a solid ~20-Watts RMS of Clean-Power, with no-Clipping,
into a single 8-Ohm-Speaker.
This setup will also drive a second 8-Ohm-Speaker, ( 4-Ohm-Total-Load ),
at the same time, while producing very close to ~40-Watts of RMS-Power.

You will also need an Aluminum-Box to build it in,
and to provide the necessary Heat-Sinking to keep it cool and alive.
.
.
.
Really appreciate these tips, however, I am a bit strapped for resources and can only work with what I have. Any other suggestions going off of the sims and circuits I sent would help a mil
 

Audioguru again

Joined Oct 21, 2019
6,758
Audio output power should always be stated in RMS continuous power. Cheap amplifiers say Peak or Maximum power which is simply double the real power and only for very short durations.

Most people play an amplifier at a volume that is half or less its continuous RMS rating to avoid any clipping distortion.
Most people play an electric guitar at a very high output producing severe distortion.

The older LM1875:
1) With a 50V supply, its output into 8 ohms is 25 continuous RMS W with noticeable 1% distortion if the supply voltage does not drop.
Then its output with low distortion (no clipping) will be about 20 continuous RMS W to 22 continuous RMS W.
Its heating at 25 continuous RMS W will be about 20W which needs a fairly large heatsink.
 

crutschow

Joined Mar 14, 2008
34,680
the lm1875 is pumping out peak 25 watts, and the load is pumping peak 52 watts,
Post your schematic.

If you using a single supply voltage, are you coupling the output with a large capacitor (e.g. about a 1,000µF) to block the DC?
If not, that's where the large load power is coming from.
 
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