PL2000 preamp square wave ringing.

marcok

Chief Journeyman
Joined
Jun 9, 2014
Messages
1,005
Location
Milan Italy
Tagline
I'm very curious about any tech item
#81

George S.

Veteran and General Yakker
Joined
Feb 18, 2020
Messages
4,639
#82
I get it Mark.
Think I'd explore if a seperate headphone amp could be inserted after the PL2000.
Then I'd look at my garage system Marantz 3200 and see how they did it there.
Then decide course of action.
Probably try to do it internal to the PL2000 for the Mojo factor.
Wish I could have kept the power supply internal to the pre, but the numbers said go external.
I see no reason that good coax wouldn't be appropriate as it works great on the amps and test equipment.
 

marcok

Chief Journeyman
Joined
Jun 9, 2014
Messages
1,005
Location
Milan Italy
Tagline
I'm very curious about any tech item
#83
Marantz 3200 preamp has no internal headphone amp ; it uses the power amp connected to output
jack via 2/3 W resistors .
Generally headphone amps use medium power transistors ( BD 139 / 140 ) , but
I think you need to redesign the PSU .
You could copy the C4000 schematic .
Ciao
Marco
 
Joined
Mar 22, 2023
Messages
103
#84
I added The Headphone jack to my new board design ( but it does not mute the line levels, since most people would just power off the main amplifiers). The four 10W power amps for the stereo and Ambience outputs are muted when NO headphone jack, or speaker jacks are plugged into. Dedicated +/- V regulators are shut down electronically with no relays, then... the 3 second mute circuit is enabled (relays) during a jack insertion to eliminate any noises.
Last night I listened to the 4 channels with the ambience circuit for the first time, using nice bookshelf speakers and the four 10W internal power amplifiers. Sounds amazing really. More than enough level for a bedroom, and really sounds super great at low levels with the Bass turnover freq switches set for more low end and low bass EQ on. Super clean. I'll be releasing boards, kits and such this summer. If interested in building this, e-mail me at ab2kc@optonline.net for more info and so I can plan a board purchase. Blank Board sets will be around $250 bucks, with a parts list, and support. 7 PC Board sets, will be fabricated on one PCB material. PCB assembly boards are separated with perforated holes to separate the boards.
1. Main Preamp PCB
2. Main Power supply PCB
3. Rear Input / Output PCB.
4. Precision Peak detector / Bar graph Display PCB
5. Headphone PCB
6. Source / Internal selector Switch PCB
7. Four 10W Power amplifier PCB
 

mlucitt

Veteran and General Yakker
Joined
Jun 24, 2011
Messages
3,397
Location
Jacksonville, FL
#86
I use a small external headphones power amp for my two PL 4000 preamps , connected to tape 2 out .
Using Tape 1 or Tape 2 without an external headphone amp does not give me volume control.
If I intercept the "Front" Left and Right signals before they get to the RCA Jacks on the back and route them to a Panel-mount Switched Stereo Headphone Jack and then back to the RCA Jacks on the back, I am thinking that might work.
A jack like this:
https://tinyurl.com/56ev8wa5
I am wondering about impedance matching with my 600 Ohm Sennheiser headphones.
 

marcok

Chief Journeyman
Joined
Jun 9, 2014
Messages
1,005
Location
Milan Italy
Tagline
I'm very curious about any tech item
#87
If you use the PL 4000 front jack output with a 600 Ohm headphone ( tested with HD 414 )
the signal is too low , because there two 1,2 kOhm resistors in series .
The reason is to protect the two output transistors ;
this output was intended as "aux" .
According the service manual rev. 6-81 you have to replace the 2 resistors with 15 Ohm
to have a correct level , but you have no the "speaker off " function .
For me using a jack like Switchcraft 14 B with a PL 400 or PL 700 is too dangerous
due the transients when you insert the jack . In other words you must ALWAYS reduce the level before
activating headphones .
I hope I was clear
Ciao
Marco
 

mlucitt

Veteran and General Yakker
Joined
Jun 24, 2011
Messages
3,397
Location
Jacksonville, FL
#88
Marco,
Thank you for that explanation. The PL2000 does not have any output transistors. The Left and Right audio output comes directly from a Quad OPAMP (Z3). This was an RC4136 (14 pin DIP), now an SMD pair of Texas Instruments OPA2134 using a 060601 Brown Dog Adapter in a new gold-plated DIP Socket. Then the output signal goes through DC Blocking Capacitors to the RCA Output Jacks. I will cut the traces between Blocking Capacitors and the RCA Jacks to wire in the Headphone Jack. The Service Manual states the maximum output (at clipping) of the PL2000 is 10V into 50K Ohms at 1kHz, that should be enough to power the Sennheiser HD600 headphones.

I don't know how to compute the voltage drop when going from 50K Ohms to 600 Ohms at 1 kHz in an AC circuit...

I would never turn on the amplifier if I was listening to headphones through the preamplifier. And I always unplug the headphones before touching any power switches.
 

George S.

Veteran and General Yakker
Joined
Feb 18, 2020
Messages
4,639
#91
I like Rod Elliott's projects. I have built several including an stand alone Headphone Amplifier - P113.
https://sound-au.com/project113.htm
I used the best OPAMP I could find at the time, a Texas Instruments LME49860. It does sound good. I get 250mW output with the 600 Ohm Headphones.
Never built any of his projects, but I've printed quite a bit of his site.
He explains things at a level I can understand.
Sitting here playing with the main system PL2000S2 feeding it various waves, amplitudes, and frequencies.
Running the volume up and down, and watching the output on the scope.
Running it off the bench supply and cable I recently built.
Looks really good from DC out to about 35 kHz. How far should it go? I don't know.
Going to break and do a bicycle ride over the local paths, then probably do some more reading about square wave testing before I reconfigure the setup for the QuantAsylum.
It does handle a sine way flawlessly out to 40 kHz with a slight attenuation of amplitude internally. The sine wave is flawless under magnification.
Later!
 

marcok

Chief Journeyman
Joined
Jun 9, 2014
Messages
1,005
Location
Milan Italy
Tagline
I'm very curious about any tech item
#92
O.K.
LME 49860 specs says : Easily drives 600 Ohm loads and Output short circuit protection .
If you use this Op Amp I agree with you .
I' m not sure that OPA 2134 is able to drive "easily a 600 Ohm load " and it has
a good short protection .
Ciao
Marco
 

J!m

Veteran and General Yakker
Joined
Dec 24, 2019
Messages
9,543
Location
Connecticut
Tagline
BOT
#93
For stand alone headphone amp, the Wyn Palmer design is excellent.

There is a thread on AK and boards may be available.

The only change I made was from a pot to a stepped attenuator and that was HUGE for sound quality clarity. Otherwise, follow the build and it’ll do very nicely. It drives difficult headphones easily as well- Wyn tested on some he personally has and no one who has built one has had any issues.
 

Gepetto

Administrator
Staff member
Joined
May 15, 2011
Messages
13,581
Location
Sterling, MA
Tagline
Old 'Arn Enthusiast
#94
O.K.
LME 49860 specs says : Easily drives 600 Ohm loads and Output short circuit protection .
If you use this Op Amp I agree with you .
I' m not sure that OPA 2134 is able to drive "easily a 600 Ohm load " and it has
a good short protection .
Ciao
Marco
Of course it can, it is spec'd at 600 ohm load. And all op amps are short circuit protected...
 
Top