I’ve been studying the past couple of months some ways to improve the field coil performance and in a way to make my tools better, to sharpen them. My goal is to get the most flux density in gap for a given current in field coil.
I tried many magnetic circuits meant to take advantage of the shape of a field coil because being a solenoid it will work better having a length per diameter ratio above unity. Works better means a more uniform field in the core.
I managed to get a simulated result of 1.5T in a 1mm wide and 12mm high gap using standard 1010 steel and a field coil consuming 7.231 Watts!
The field coil is made with 1 mm EC wire so it will be able to handle a lot more current. You will get more flux density this way but the curve will get more and more bell-shaped.
As you can see for 1 Amp you get a very nice increase in flux density. Its not as linear but the difference is small.
There is a balance, hard to achieve, between linearity and maximizing flux density. And this has to do with the magnetic circuit and saturating the pole plates. I usually want to saturate them right at the gap. This way it’s less prone to modulation.
In placing the saturation area, the central pole piece and the top plate geometries have a large role. Always remember that the thin parts of steel saturate faster and zones with transitions from one dimension to another or sharp angles are also prone to saturation.
Below you can see a motor with a linear flux density curve over a 20 mm travel. The central pole piece is of the same diameter as before. You can see that making the top plate thicker killed the saturation around the gap. To bring it back, we must use a larger diameter for a voice coil. But more on that in a future article.
Do you remember BOSS Slow Gear pedal? If your a guitarist you most likely do or at least you’ve heard of it. It was a great pedal sold from 1979 to 1982 and it was made in Japan. The pedal would cut the attack of your notes giving a swelling sound. It god famous for making the guitar sound kinda like a violin.
I always liked that effect and i even made a clone a few years back. It is based on a 2SK30 JFET and it was a pain getting these transistors. It was a lot of fun though and i though i should make a Project Ryu swell effect pedal and so LAGGER was born!
Recently i worked on a few projects with LM13600/LM13700, one of them is a nice noise gate / compressor unit which i will present at a later date, and i really like the VCAs that can be built with these chips.
To cut the attack of a note and then swell the volume basically we need a triggered fade in effect. This means that we need to control our VCA with a rising voltage using what i call a ramp generator.
Below you can see the block diagram of the Lagger:
The input is fed into an ADC channel to be rectified and averaged in order to detect when a note is played. Once it is detected, the ramp generator is triggered and provides the control voltage for the first VCA.
Since LM13600/LM13700 is a dual amplifier the second one is configured as a VCA with manually set control voltage. In the picture below you can see how the circuit works. The top signal is the input signal, the middle signal is the output of the ramp generator and the bottom signal is the trigger.
There is a problem with using the ramp generator circuit this way. The capacitor is discharged too quickly when the trigger is interrupted and this causes an audible thump noise when trigger goes off. Looking below at the schematic we can see the discharge current goes through CE junction of Q1.
We can lower this current by inserting a resistor between ground and Q1’s emitter but in our specific application that will cause an offset and the output will not be totally silent in absence of input signal.
Another way to solve the problem is by paralleling a capacitor with R3 (Q2’s emitter resistor) This will cause a fade out effect and eliminate the thump noise.
Below you can find the schematic for the Lagger:
U5 shows as TL071 but you need an opamp with higher output current sink capability. Something like HA17358 with 50mA capability is good:
Trigger for the ramp generated is created when the microcontroller detects a signal from guitar. In my last article i have explained a way to rectify and average an analog signal using ADC and software. If the input level is higher than a set threshold level then ramp generator is triggered.
In the first units the middle pot was used to set a sustain period but that was changed to sensitivity control as it proved to be much more helpful.
J1 is a push-button which will generate an interrupt for the microcontroller and provide a true bypass via the SPDT relay.
You will notice some unusual supply voltages. For example the microcontroller’s Vdd is set to GND and Vss to -5V. This is done in order to provide correct trigger levels and avoid using other active components to shift the level.
Below you can see the PCB for the unit:
Here are some pictures with Project Ryu Lagger:
Here is a short video with the unit in action:
I will be supplying the hex file for the PIC18f1320 microcontroller in my next newsletters so if you want to built the unit and your not a subscriber yet please use the top right form to subscribe.
Also in my newsletter you will find offer for kits and complete units for those who don’t do well with electronics.
Recently i have finished a pair of field coil mid-bass drivers. The driver has a Fs of 50Hz and a frequency response up to 7kHz. It is a dual voice coil design features a 160mm diameter motor with a max of 1.4T in the gap, Ryu spider, non-treated paper cone, triple roll cloth surround and bullet shaped phase plug.
The voice coil construction was presented in an earlier article here. Inside the gap set at around 1T flux density, the voice coil inductance was measured at 0.52mH @1kHz and 0.15mH@10kHz.
The high compliance of the Ryu spider was measured at around 3.4mm/N for this mid-bass and makes the compliance of the cloth surround the determinant factor in the system’s total compliance value.
Below you can see the impedance measurement and frequency response of these mid-bass drivers. The frequency response is measured with the voice coils connected in parallel. you can see a nice constant 6db/octave rise in response. You can use this characteristic when designing the loudspeaker if you consider baffle step or horn loading.
I have tested the unit and played Isao Tomita’s Snowflakes are Dancing and The Police Synchronicity. The sound is thin with the voice coils connected in parallel and no baffle but with the voice coils in series even without a baffle you can reach a good tonal balance. This indicates the unit can be used successfully in an Open Baffle system or in a horn loaded enclosure with the voice coils in parallel.
In this article i will present a simple delay circuit that will be used to couple the speakers to the amplifier after a certain settling time was allowed. The circuit also allows for to be controlled by an external 5V logic signal. This can be used to decouple the speakers in case a fault is detected.
The circuit schematic is presented in figure 1 and as you can see it uses just discrete components. It is a linear voltage ramp generator that commands a power transistor. The current charging capacitor C1 and the capacitor’s value are the parameters that set the ramp’s slope.
In figure 1 Q3 forms a constant current source adjustable via POT1. R1, R2, D1, D2 set a voltage on the base of Q3 of about 5.4V and this means about 6V voltage drop over R6 and POT1 series connection. Assuming Ic3 = Ie3=Icharge,
Icharge = 6V/(R6+POT1)
Lets set POT1 at 90kohms for ease of calculation. This gives R6+POT1 = 100k.
Icharge = 60uA
Since Q3 is in saturation mode we can assume a voltage drop over C-E of about 0.5V so the voltage over the capacitor Vc1= 5.5V. The time for the capacitor to be charged to 5.5V is defined by the below equation:
T= (C1*Vc1)/Icharge = 0.91 second
Q2 buffers the voltage across C1 capacitor. It also provides a small delay until Vc1 reaches around 0.6V to bias Q2’s B-E junction. Q1 acts as a switch and when turned on via a 5V signal it absorbs most of the current from Q3 and capacitor will not be charged.
Q4 has the role to drive the relay. It is a small power transistor and it’s enabled via POT2. This variable transistor has the role to set the on/off steps based on the ramp voltage. If too low the relay will be on very fast and stay on if too high the relay will never activate.
In figure 2 the time step is 200ms and we can see the ramp is about 1s long, very close to what we calculated. The blue trace is the Fault signal. When a 5V pulse is present the capacitor C1 is discharged very fast (pink trace) and speakers are decoupled (green trace). When the fault signal goes to logic low or ground the ramp generator shortly starts the process and enables the relay after about 1 second.
Figure 3 shows how the relay is activated faster if the POT2 is set too low in value and figure 4 shows a correct setting. The yellow trace represents power switched on.
Below you can see the circuit in the right side of the board.
Parts list does not contain the connectors in the schematic because the circuit most likely will be used as a part of something bigger:
Today i could do a first test of the new spider for my 12 inch field-coil loudspeaker. I attached the spider to a voice coil and to a frame. Without any added mass i applied variable DC voltage on the voice coil to determine how much it will move at a specific voltage.
First I had to create a variable DC Voltage generator.This can be done with op amp but since the voice coil has low resistance (Rdc = 5.1 ohms) the op amp will need to deliver high current. This is why i chose LA6520 which has a 500mA output current. In the picture below you can see the schematic. Please note that LA6520 has different pinout like in the next picture so keep it in mind when soldering. Also this circuit needs symmetric +/-15V power supply.
The test setup includes a series resistor. We will measure the voltage drop on this resistor in order to calculate the current through the voice coil. It is advantageous to display excursion vs current as the force that pushes against the spider’s restoring force is defined by F=BL*i where BL is the motor strength factor and i is current through voice coil. The series resistor was formed from two 30W 3R3 resistors connected in parallel. This way temperature effects are negligible. Below you can see the setup.
Spider’s behavior proved to be quite linear. Shown below is the results measured for the voice coil’s up direction. Above 3.5mm the voice coil is getting out of the magnetic gap so force becomes smaller. As can be seen though Project Ryu’s magnetic circuit is strong enough to use the fringe field to its advantage increasing this limit to about 5 mm.
In a previous article I wrote about a monitoring amplifier i want to build. This weekend i could continue with this and i built the tone control circuitry. It is a Baxandall type but the values are a little atypical as i made this tone control tailored for my needs.
I wanted controls that can help in a large format 2 way loudspeakers employing a crossover frequency around 1kHz. I need a shelving filter that can gradually bring up or down that range. Below is the schematic and the graph showing the maximum boost-cut levels.
Schematic of Tone Control Circuit
Tone control range
The schematic represents 1 channel and it is not showing two 100n/63V polyester film capacitors used to decouple TL074 power rails.
List of Materials
C 100n/63V Polyester Film
C 47n Polyester
C 10n Ceramic
C 150p Ceramic
Pot Dual 33k
Pot Dual 150k
Molex Connector 8 pin
6 pin header
I didnt include the 3 pin Molex connector for input you will see in the pictures as i use it temporary to connect 2 RCA female plugs to it. Below you can see the picture of the circuit. It is a bit annoying soldering the SMD components on the prototype board but not a big problem. Since this is a high gain circuit grounding needs increased attention.
Connection between boards will be made with a 6 wire shielded cable. This cable type is often used in home security installations like alarms systems and has each of the wires individually shielded.
The shields are tied together and grounded to one side of the cable only as signal ground is carried on one or more of the six wires. Used thermoretractable tube to isolate the open shield cable end.
Using the line input of the soundcard i could measure the response of the tone control circuit. As can be seen it is very close to the predicted response. At the bass boost you can see the line getting flatter towards the end because of saturation.
Here is a picture during tests:
The bass control indeed it is just right, at low volume levels you can still get the deep bass without going through walls.
For a while, we have been discussing on diyAudio the influence of the spider. Ideally we want a low mass, low radiating area compliance for this. Also a very high stiffness towards side-to-side movement to avoid rocking modes.
To continue with Harbinger project i would like to investigate the drivers and what could be expected from them. I already mentioned two of the 15 inchers and that i decided to use a version of 8512F.
The motor on this driver is very strong 220mm in diameter, noiseless vent and T-shaped central pole piece for symmetric field in gap as seen below.
T-shaped pole piece 8512F
The original had this set of parameters:
This results in the following equivalent electrical components:
Lces -> corresponds to Cms; Cmes -> corresponds to Mms; Res -> corresponds to Rms
First modification was changing the suspension system with a more softer one. The new spider has larger deeper and fewer corrugations. I made this modification so that the woofer can perform better at lower input power.
Original spider of the 15 incher
New spider for the 15 incher
The replacement spider i used before for a 10 inch driver modification to fit in a 4″ voice coil.
At this point i needed to build a new box to put in the woofer and study it’s behavior. I built a 160 Liters vented box with 2 ports each of 72mm diameter and 140mm length.
Spice simulation of loudspeaker equivalent model with LPF
Measured impedance of 15 incher with LPF
The box wasn’t well braced and needed some strengthening but the output in the bass region was satisfactory. Even at low levels the bass was low with a good amount of punch. My room helps a lot though on the low end. I have wooden floors and ceiling and lots of furniture.
In the next article we will look at what else can be improved on the drivers and the box.